diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 47d93a3..7131f63 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -521,18 +521,26 @@ jobs:
             sleep 2
           done
 
-      # Two logins per run → reset first (see note above).
+      # Six logins per run since #180 doubled the scans (3 contexts × light/
+      # dark, limit is 10/min) → reset first (see note above).
       - name: Reset login rate limit before a11y pack
         run: |
           echo "DELETE FROM rate_limits WHERE key LIKE 'login%';" | \
             pnpm --filter @dorfteich/api exec prisma db execute --stdin --url "$DATABASE_URL"
 
-      # WCAG-A/AA-Regressionsschutz (issue #171): axe-Scan der Kernscreens.
+      # WCAG-A/AA-Regressionsschutz (issue #171): axe-Scan der Kernscreens,
+      # seit #180 in beiden Farbschemata.
       - name: Run a11y pack
         run: |
           E2E_BASE_URL=http://localhost:5173 \
             pnpm --filter @dorfteich/web exec playwright test e2e/a11y.spec.ts
 
+      # Hell/Dunkel/System-Umschalter (issue #180).
+      - name: Run theme pack
+        run: |
+          E2E_BASE_URL=http://localhost:5173 \
+            pnpm --filter @dorfteich/web exec playwright test e2e/theme.spec.ts
+
       - name: Run setup wizard pack
         run: |
           E2E_BASE_URL=http://localhost:5175 E2E_SETUP=1 \
diff --git a/apps/web/e2e/a11y.spec.ts b/apps/web/e2e/a11y.spec.ts
index 7b232d4..97f4e7c 100644
--- a/apps/web/e2e/a11y.spec.ts
+++ b/apps/web/e2e/a11y.spec.ts
@@ -10,10 +10,15 @@ import { contextForUser } from './helpers';
  * der Issues #162–#170 verletzungsfrei; jede neue Verletzung bricht den
  * Build. Best-Practice-Regeln (axe-Tag best-practice) prüfen wir hier
  * bewusst NICHT, nur normative WCAG-Kriterien.
+ *
+ * Seit issue #180 läuft jeder Scan in BEIDEN Farbschemata: emulateMedia
+ * setzt prefers-color-scheme, theme-init.js löst den Default „System“ zum
+ * konkreten data-theme auf, axe misst dann die echten Dark-Token-Farben.
  */
 
 const BASE = process.env.E2E_BASE_URL ?? 'http://localhost:5173';
 const TAGS = ['wcag2a', 'wcag21a', 'wcag2aa', 'wcag21aa'];
+const SCHEMES = ['light', 'dark'] as const;
 
 /** Bewusst tolerierte Regel-IDs — nur mit Begründung ergänzen. */
 const ALLOWED_RULES: string[] = [];
@@ -32,42 +37,52 @@ async function expectClean(page: Page, label: string): Promise<void> {
   ).toEqual([]);
 }
 
-test('login page passes the axe WCAG A/AA scan', async ({ page }) => {
-  await page.goto('/login');
-  await page.waitForLoadState('networkidle');
-  await expectClean(page, '/login');
-});
+for (const scheme of SCHEMES) {
+  test.describe(`${scheme} scheme`, () => {
+    test(`login page passes the axe WCAG A/AA scan (${scheme})`, async ({ page }) => {
+      await page.emulateMedia({ colorScheme: scheme });
+      await page.goto('/login');
+      await page.waitForLoadState('networkidle');
+      await expectClean(page, `/login (${scheme})`);
+    });
 
-test('reading and editing a page passes the axe WCAG A/AA scan', async ({ browser }) => {
-  const context = await contextForUser(browser, BASE, 'fixture-user');
-  const page = await context.newPage();
-  await page.goto('/p/content-fixtures/every-element');
-  await page.waitForLoadState('networkidle');
-  await expectClean(page, 'Lesemodus every-element');
+    test(`reading and editing a page passes the axe WCAG A/AA scan (${scheme})`, async ({
+      browser,
+    }) => {
+      const context = await contextForUser(browser, BASE, 'fixture-user');
+      const page = await context.newPage();
+      await page.emulateMedia({ colorScheme: scheme });
+      await page.goto('/p/content-fixtures/every-element');
+      await page.waitForLoadState('networkidle');
+      await expectClean(page, `Lesemodus every-element (${scheme})`);
 
-  await page.locator('.editor-page__mode-toggle').click();
-  await page.locator('.ProseMirror[contenteditable="true"]').waitFor({ timeout: 10_000 });
-  await page.waitForTimeout(500);
-  await expectClean(page, 'Editor every-element');
-  await context.close();
-});
+      await page.locator('.editor-page__mode-toggle').click();
+      await page.locator('.ProseMirror[contenteditable="true"]').waitFor({ timeout: 10_000 });
+      await page.waitForTimeout(500);
+      await expectClean(page, `Editor every-element (${scheme})`);
+      await context.close();
+    });
 
-test('user settings pass the axe WCAG A/AA scan', async ({ browser }) => {
-  const context = await contextForUser(browser, BASE, 'fixture-user');
-  const page = await context.newPage();
-  await page.goto('/settings');
-  await page.waitForLoadState('networkidle');
-  await expectClean(page, '/settings');
-  await context.close();
-});
+    test(`user settings pass the axe WCAG A/AA scan (${scheme})`, async ({ browser }) => {
+      const context = await contextForUser(browser, BASE, 'fixture-user');
+      const page = await context.newPage();
+      await page.emulateMedia({ colorScheme: scheme });
+      await page.goto('/settings');
+      await page.waitForLoadState('networkidle');
+      await expectClean(page, `/settings (${scheme})`);
+      await context.close();
+    });
 
-test('admin area passes the axe WCAG A/AA scan', async ({ browser }) => {
-  const context = await contextForUser(browser, BASE, 'fixture-admin');
-  const page = await context.newPage();
-  await page.goto('/admin');
-  await page.waitForLoadState('networkidle');
-  // Personenliste sichtbar, inkl. der Icon-Aktionen (issue #175).
-  await page.locator('.user-manager__table .user-row').first().waitFor();
-  await expectClean(page, '/admin');
-  await context.close();
-});
+    test(`admin area passes the axe WCAG A/AA scan (${scheme})`, async ({ browser }) => {
+      const context = await contextForUser(browser, BASE, 'fixture-admin');
+      const page = await context.newPage();
+      await page.emulateMedia({ colorScheme: scheme });
+      await page.goto('/admin');
+      await page.waitForLoadState('networkidle');
+      // Personenliste sichtbar, inkl. der Icon-Aktionen (issue #175).
+      await page.locator('.user-manager__table .user-row').first().waitFor();
+      await expectClean(page, `/admin (${scheme})`);
+      await context.close();
+    });
+  });
+}
diff --git a/apps/web/e2e/settings-nav.spec.ts b/apps/web/e2e/settings-nav.spec.ts
index aebfdaf..2c908e9 100644
--- a/apps/web/e2e/settings-nav.spec.ts
+++ b/apps/web/e2e/settings-nav.spec.ts
@@ -28,8 +28,8 @@ test('user settings show the jump nav and clicking scrolls + activates', async (
   await expect(nav).toBeVisible();
   const links = nav.locator('.settings-nav__link');
   // Profile, password, sessions, watches, API tokens, feed tokens, data export.
-  // 8 seit #170 (neue Bedienungs-Sektion).
-  await expect(links).toHaveCount(8);
+  // 8 seit #170 (Bedienung), 9 seit #180 (Erscheinungsbild).
+  await expect(links).toHaveCount(9);
 
   // Jump to the last section: it scrolls into view and becomes active.
   const last = links.last();
diff --git a/apps/web/e2e/theme.spec.ts b/apps/web/e2e/theme.spec.ts
new file mode 100644
index 0000000..0dc4e9e
--- /dev/null
+++ b/apps/web/e2e/theme.spec.ts
@@ -0,0 +1,66 @@
+import { expect, test, type Page } from '@playwright/test';
+
+import { contextForUser } from './helpers';
+
+/**
+ * Theme-Umschalter (issue #180): die Hell/Dunkel/System-Wahl in den
+ * Einstellungen wirkt sofort, überlebt den Reload (localStorage) und folgt
+ * im System-Modus Live-Änderungen des OS-Schemas. Klassen-Hooks statt
+ * lokalisierter Texte (UI-Sprache folgt dem Profil-Locale).
+ */
+
+const BASE = process.env.E2E_BASE_URL ?? 'http://localhost:5173';
+
+const radio = (page: Page, value: string) =>
+  page.locator(`.settings-fieldset input[value="${value}"]`);
+
+const effectiveTheme = (page: Page) => page.evaluate(() => document.documentElement.dataset.theme);
+
+test('theme choice applies instantly, persists, and system mode follows the OS', async ({
+  browser,
+}) => {
+  const context = await contextForUser(browser, BASE, 'fixture-user');
+  const page = await context.newPage();
+  await page.emulateMedia({ colorScheme: 'light' });
+  await page.goto('/settings');
+  await page.locator('.settings-fieldset').waitFor();
+
+  // Default: System, auf einem hellen OS also light.
+  await expect(radio(page, 'system')).toBeChecked();
+  expect(await effectiveTheme(page)).toBe('light');
+
+  // Explizit Dunkel: wirkt sofort, ohne Reload, trotz hellem OS.
+  await radio(page, 'dark').check();
+  expect(await effectiveTheme(page)).toBe('dark');
+  await expect(page.locator('meta[name="theme-color"]').first()).toHaveAttribute(
+    'content',
+    '#10161d',
+  );
+
+  // Persistenz: Wahl und Theme überleben den Reload (theme-init.js liest
+  // denselben localStorage-Key vor dem ersten Paint).
+  await page.reload();
+  await page.locator('.settings-fieldset').waitFor();
+  await expect(radio(page, 'dark')).toBeChecked();
+  expect(await effectiveTheme(page)).toBe('dark');
+  expect(await page.evaluate(() => window.localStorage.getItem('ui.theme.mode'))).toBe('"dark"');
+
+  // Zurück auf System: folgt dem hellen OS …
+  await radio(page, 'system').check();
+  expect(await effectiveTheme(page)).toBe('light');
+
+  // … und Live-Wechseln des OS-Schemas ohne Interaktion.
+  await page.emulateMedia({ colorScheme: 'dark' });
+  await expect
+    .poll(async () => effectiveTheme(page), { message: 'system mode follows OS change' })
+    .toBe('dark');
+  await page.emulateMedia({ colorScheme: 'light' });
+  await expect.poll(async () => effectiveTheme(page)).toBe('light');
+
+  // Explizite Wahl gewinnt gegen das OS-Schema.
+  await page.emulateMedia({ colorScheme: 'dark' });
+  await radio(page, 'light').check();
+  expect(await effectiveTheme(page)).toBe('light');
+
+  await context.close();
+});
diff --git a/apps/web/index.html b/apps/web/index.html
index 64aa490..284207c 100644
--- a/apps/web/index.html
+++ b/apps/web/index.html
@@ -3,7 +3,17 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <!-- Applies at parse time, before any script or stylesheet: the UA
+         canvas is already dark for system-dark users, killing the white
+         flash. theme-init.js then narrows it to the resolved theme. -->
+    <meta name="color-scheme" content="light dark" />
+    <meta name="theme-color" media="(prefers-color-scheme: light)" content="#2f6f4f" />
+    <meta name="theme-color" media="(prefers-color-scheme: dark)" content="#10161d" />
     <title>Dorfteich</title>
+    <!-- Classic (non-module) script: executes during head parsing, before
+         first paint and before the deferred module bundle. External file
+         because the prod CSP forbids inline scripts (issue #180). -->
+    <script src="/theme-init.js"></script>
     <!-- Self-hosted catalog @font-face rules (ADR 0016), baked into the image
          by deploy/fonts/build-fonts.mjs. Absent in a plain dev build → the app
          falls back to system fonts; never references a third-party origin. -->
diff --git a/apps/web/public/theme-init.js b/apps/web/public/theme-init.js
new file mode 100644
index 0000000..aa1e8c9
--- /dev/null
+++ b/apps/web/public/theme-init.js
@@ -0,0 +1,34 @@
+/* Pre-paint theme boot (issue #180). Must stay a plain classic script in a
+ * separate file: the prod CSP (nginx.conf, script-src 'self') forbids inline
+ * scripts, and executing during <head> parsing — before the deferred module
+ * bundle — is what prevents a white flash for dark users. Reads the same
+ * JSON-encoded localStorage keys as src/theme/theme.ts; keep them in sync. */
+(function () {
+  var theme = 'light';
+  try {
+    var raw = window.localStorage.getItem('ui.theme.mode');
+    var mode = raw === null ? 'system' : JSON.parse(raw);
+    var dark =
+      mode === 'dark' ||
+      (mode !== 'light' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+    if (dark) theme = 'dark';
+  } catch {
+    /* Broken storage/JSON: default to light. */
+  }
+  document.documentElement.dataset.theme = theme;
+  document.documentElement.style.colorScheme = theme;
+  try {
+    /* Hook for the accent-theming stage (ADR 0018): a pre-derived stylesheet
+     * cached under ui.theme.css is injected before first paint so custom
+     * accents do not flash either. Unset until that stage ships. */
+    var css = window.localStorage.getItem('ui.theme.css');
+    if (css) {
+      var style = document.createElement('style');
+      style.id = 'user-theme';
+      style.textContent = JSON.parse(css);
+      document.head.appendChild(style);
+    }
+  } catch {
+    /* Optional enhancement only. */
+  }
+})();
diff --git a/apps/web/src/main.tsx b/apps/web/src/main.tsx
index 7d4c714..e8aa9d1 100644
--- a/apps/web/src/main.tsx
+++ b/apps/web/src/main.tsx
@@ -8,9 +8,16 @@ import { AuthProvider } from './auth/auth-context';
 import { ToastProvider } from './components/Toast';
 import './i18n';
 import { ApiError } from './lib/api';
+import { applyTheme, initSystemThemeListener, readStoredThemeMode } from './theme/theme';
 import './styles/tokens.css';
 import './styles/base.css';
 
+// theme-init.js already themed the document pre-paint; re-applying here is
+// a no-op safety net for contexts serving index.html without it, and the
+// listener keeps 'system' users in sync with live OS scheme changes.
+applyTheme(readStoredThemeMode());
+initSystemThemeListener();
+
 const queryClient = new QueryClient({
   defaultOptions: {
     queries: {
diff --git a/apps/web/src/pages/SettingsPage.tsx b/apps/web/src/pages/SettingsPage.tsx
index 2f92ff9..d72e4e9 100644
--- a/apps/web/src/pages/SettingsPage.tsx
+++ b/apps/web/src/pages/SettingsPage.tsx
@@ -17,6 +17,7 @@ import { WatchesSection } from '../watches/WatchesSection';
 import { useDocumentTitle } from '../lib/use-document-title';
 import { SINGLE_KEY_SHORTCUTS_KEY } from '../lib/single-key-shortcuts';
 import { usePersistentState } from '../lib/use-persistent-state';
+import { applyTheme, THEME_MODE_KEY, type ThemeMode } from '../theme/theme';
 interface SessionView {
   id: string;
   createdAt: string;
@@ -38,6 +39,7 @@ export function SettingsPage(): React.JSX.Element {
         <WatchesSection />
         <ApiTokensSection />
         <FeedTokensSection />
+        <AppearanceSection />
         <InteractionSection />
         <DataExportSection />
       </SettingsLayout>
@@ -45,6 +47,43 @@ export function SettingsPage(): React.JSX.Element {
   );
 }
 
+/** Erscheinungsbild (issue #180): Hell/Dunkel/System — wie
+ * InteractionSection eine lokale Geräte-Einstellung, kein Server-Zustand. */
+function AppearanceSection(): React.JSX.Element {
+  const { t } = useTranslation();
+  const [mode, setMode] = usePersistentState<ThemeMode>(THEME_MODE_KEY, 'system');
+  const choose = (value: ThemeMode): void => {
+    setMode(value);
+    applyTheme(value);
+  };
+  const options: { value: ThemeMode; label: string }[] = [
+    { value: 'light', label: t('settings:appearance.light') },
+    { value: 'dark', label: t('settings:appearance.dark') },
+    { value: 'system', label: t('settings:appearance.system') },
+  ];
+  return (
+    <section className="settings-section">
+      <h2>{t('settings:appearance.title')}</h2>
+      <fieldset className="settings-fieldset">
+        <legend>{t('settings:appearance.legend')}</legend>
+        {options.map((option) => (
+          <label key={option.value} className="settings-checkbox">
+            <input
+              type="radio"
+              name="theme-mode"
+              value={option.value}
+              checked={mode === option.value}
+              onChange={() => choose(option.value)}
+            />
+            {option.label}
+          </label>
+        ))}
+      </fieldset>
+      <p className="field__hint">{t('settings:appearance.hint')}</p>
+    </section>
+  );
+}
+
 /** Bedienungs-Einstellungen (issue #170, WCAG 2.1.4): Einzeltasten-Kürzel
  * abschaltbar machen — lokale Geräte-Einstellung, kein Server-Zustand. */
 function InteractionSection(): React.JSX.Element {
diff --git a/apps/web/src/styles/base.css b/apps/web/src/styles/base.css
index a4b0538..00b8407 100644
--- a/apps/web/src/styles/base.css
+++ b/apps/web/src/styles/base.css
@@ -1401,6 +1401,8 @@ button {
   margin-left: -0.4rem;
   border: 2px solid var(--color-bg);
   border-radius: 50%;
+  /* Deliberately NOT a theme token: USER_COLORS backgrounds are chosen so
+     white text clears 4.5:1 (user-color.test.ts) independent of the theme. */
   color: #ffffff;
   font-size: 0.7rem;
   font-weight: 600;
@@ -1443,6 +1445,7 @@ button {
   left: -1px;
   padding: 0.05rem 0.3rem;
   border-radius: 3px 3px 3px 0;
+  /* White on USER_COLORS, theme-independent — see .presence-avatar. */
   color: #ffffff;
   font-size: 0.7rem;
   font-weight: 600;
@@ -1909,6 +1912,9 @@ ul[data-type='task_list'] li p:last-of-type {
   font-size: 0.72rem;
   line-height: 1.5;
   white-space: nowrap;
+  /* Invisible on light; separates arbitrary user label colors from the
+     dark canvas (issue #180). */
+  border: 1px solid var(--color-chip-outline);
 }
 
 .label-chip__swatch {
@@ -2210,7 +2216,7 @@ ul[data-type='task_list'] li p:last-of-type {
 }
 
 .member-seats__item {
-  background: var(--color-surface-muted, rgba(127, 127, 127, 0.12));
+  background: var(--color-surface-muted);
   border-radius: var(--radius-sm, 0.375rem);
   padding: var(--space-1) var(--space-2);
   font-size: 0.875rem;
@@ -3007,7 +3013,7 @@ ul[data-type='task_list'] li p:last-of-type {
   border-radius: var(--radius-sm, 0.375rem);
   padding: var(--space-3);
   margin-bottom: var(--space-3);
-  background: var(--color-surface-muted, rgba(127, 127, 127, 0.06));
+  background: var(--color-bg-subtle);
 }
 
 .attachments-panel__header {
@@ -3197,23 +3203,23 @@ ul[data-type='task_list'] li p:last-of-type {
   padding: 0.1rem 0.5rem;
   border-radius: 999px;
   font-size: 0.8125rem;
-  background: var(--color-surface-muted, #e2e8f0);
+  background: var(--color-surface-muted);
   margin-left: var(--space-1);
 }
 
 .system-badge--ok {
-  background: #d6f2df;
-  color: #1d6f42;
+  background: var(--color-badge-ok-bg);
+  color: var(--color-badge-ok-text);
 }
 
 .system-badge--error {
-  background: #fbe3e0;
-  color: #a02818;
+  background: var(--color-badge-error-bg);
+  color: var(--color-badge-error-text);
 }
 
 .system-badge--warn {
-  background: #fdf0d4;
-  color: #8a5a00;
+  background: var(--color-badge-warn-bg);
+  color: var(--color-badge-warn-text);
 }
 
 .system-jobs__notice {
@@ -3312,11 +3318,11 @@ ul[data-type='task_list'] li p:last-of-type {
 .button--danger {
   border-color: var(--color-danger);
   background: var(--color-danger);
-  color: var(--color-accent-contrast);
+  color: var(--color-danger-contrast);
 }
 
 .button--danger:hover:not(:disabled) {
-  background: #8a0718;
+  background: var(--color-danger-strong);
 }
 
 /* Pond creation form in the switcher + pond danger zone */
@@ -3527,7 +3533,7 @@ ul[data-type='task_list'] li p:last-of-type {
 .comments-section__count {
   margin-left: var(--space-2);
   font-size: 0.8125rem;
-  background: var(--color-surface-muted, #e2e8f0);
+  background: var(--color-surface-muted);
   border-radius: 999px;
   padding: 0.05rem 0.5rem;
 }
@@ -3604,7 +3610,7 @@ ul[data-type='task_list'] li p:last-of-type {
   background: none;
   border: none;
   padding: 0;
-  color: var(--color-primary, #2f6f4f);
+  color: var(--color-accent);
   cursor: pointer;
   font-size: 0.8125rem;
   text-decoration: underline;
@@ -3641,8 +3647,8 @@ ul[data-type='task_list'] li p:last-of-type {
 
 /* Watches (issue #93) */
 .watch-toggle--active {
-  background: var(--color-primary, #2f6f4f);
-  color: #fff;
+  background: var(--color-accent);
+  color: var(--color-accent-contrast);
 }
 
 .watches-list {
@@ -3672,6 +3678,18 @@ ul[data-type='task_list'] li p:last-of-type {
   margin: var(--space-2) 0;
 }
 
+/* Radio groups in settings sections (issue #180). */
+.settings-fieldset {
+  border: none;
+  margin: 0;
+  padding: 0;
+}
+
+.settings-fieldset legend {
+  padding: 0;
+  font-weight: 600;
+}
+
 .pond-settings-page__header {
   display: flex;
   align-items: center;
@@ -3696,8 +3714,8 @@ ul[data-type='task_list'] li p:last-of-type {
   position: absolute;
   top: -2px;
   right: -4px;
-  background: #a02818;
-  color: #fff;
+  background: var(--color-danger);
+  color: var(--color-danger-contrast);
   border-radius: 999px;
   font-size: 0.7rem;
   padding: 0 0.35rem;
@@ -3929,7 +3947,7 @@ ul[data-type='task_list'] li p:last-of-type {
 }
 
 .dt-date--overdue {
-  color: #b91c1c;
+  color: var(--color-danger);
 }
 
 /* Task overview block (issue #154). */
diff --git a/apps/web/src/styles/tokens.css b/apps/web/src/styles/tokens.css
index 380f34e..fa49081 100644
--- a/apps/web/src/styles/tokens.css
+++ b/apps/web/src/styles/tokens.css
@@ -4,6 +4,10 @@
  * one-line changes. The look is deliberately plain and professional.
  */
 :root {
+  /* Native controls/scrollbars follow the active theme; the dark block
+     below flips this together with the palette (issue #180). */
+  color-scheme: light;
+
   /* Font slots per ADR 0016; the real self-hosted fonts arrive with issue #66. */
   --font-heading: 'Roboto', system-ui, -apple-system, 'Segoe UI', sans-serif;
   --font-body: 'Roboto', system-ui, -apple-system, 'Segoe UI', sans-serif;
@@ -21,6 +25,9 @@
    * token with a #fff fallback; the fallback-less ones dropped their
    * background entirely (#120). */
   --color-surface: #ffffff;
+  /* Muted pill/badge surfaces (member seats, count pills, neutral badges).
+     Was a fallback-only value scattered through base.css before #180. */
+  --color-surface-muted: #e2e8f0;
   --color-border: #d9e2ec;
   /* Boundaries of interactive controls (inputs, selects) need 3:1 against
      both backgrounds (WCAG 1.4.11) — #d9e2ec is only ~1.3:1. */
@@ -28,11 +35,28 @@
   --color-accent: #2f6f4f;
   --color-accent-contrast: #ffffff;
   --color-danger: #ab091e;
+  /* Text sitting ON a danger background (buttons, notification badge) —
+     paired with --color-danger the same way --color-accent-contrast is
+     paired with --color-accent; the two must flip together per theme. */
+  --color-danger-contrast: #ffffff;
+  --color-danger-strong: #8a0718;
   --color-ok: #14803c;
   /* Favorite stars and tree icons (issue #132) — a readable gold. ICON
      use only: 3.25:1 passes the 3:1 non-text minimum but NOT the 4.5:1
      required for text (audit A11Y-024c). */
   --color-favorite: #b8860b;
+  /* Status badges (admin system page): pastel surface + strong text, each
+     pair ≥ 4.5:1 in both themes (asserted by theme-contrast.test.ts). */
+  --color-badge-ok-bg: #d6f2df;
+  --color-badge-ok-text: #1d6f42;
+  --color-badge-error-bg: #fbe3e0;
+  --color-badge-error-text: #a02818;
+  --color-badge-warn-bg: #fdf0d4;
+  --color-badge-warn-text: #8a5a00;
+  /* Separation ring for user-colored label chips/swatches — invisible on
+     light, a faint light outline on dark where arbitrary label colors can
+     melt into the background. */
+  --color-chip-outline: transparent;
 
   /* Spacing scale (rem-based). */
   --space-1: 0.25rem;
@@ -46,3 +70,36 @@
   --topbar-height: 3rem;
   --sidebar-width: 16rem;
 }
+
+/*
+ * Dark palette (issue #180). public/theme-init.js and theme/theme.ts always
+ * resolve the user's Light/Dark/System choice to a CONCRETE data-theme on
+ * <html> — there is deliberately no @media fallback here, so this single
+ * selector is the whole contract for later theming stages (ADR 0018).
+ * Every pair is contrast-asserted by theme-contrast.test.ts; border-input
+ * and favorite pass ≥ 3:1 against both palettes and stay unchanged.
+ */
+:root[data-theme='dark'] {
+  color-scheme: dark;
+
+  --color-text: #e2e8f0;
+  --color-text-muted: #a7b3c0;
+  --color-bg: #10161d;
+  --color-bg-subtle: #161d26;
+  --color-surface: #1b2430;
+  --color-surface-muted: #26303c;
+  --color-border: #323e4d;
+  --color-accent: #5cb88a;
+  --color-accent-contrast: #10161d;
+  --color-danger: #f87171;
+  --color-danger-contrast: #10161d;
+  --color-danger-strong: #fa8f8f;
+  --color-ok: #4ade80;
+  --color-badge-ok-bg: #1b3d2b;
+  --color-badge-ok-text: #8fdcb0;
+  --color-badge-error-bg: #43181d;
+  --color-badge-error-text: #f3a8a8;
+  --color-badge-warn-bg: #3c3113;
+  --color-badge-warn-text: #e3c06b;
+  --color-chip-outline: rgba(255, 255, 255, 0.18);
+}
diff --git a/apps/web/src/theme/theme-contrast.test.ts b/apps/web/src/theme/theme-contrast.test.ts
new file mode 100644
index 0000000..3236e24
--- /dev/null
+++ b/apps/web/src/theme/theme-contrast.test.ts
@@ -0,0 +1,100 @@
+import { readFileSync } from 'node:fs';
+
+import { describe, expect, it } from 'vitest';
+
+/**
+ * Mechanical contrast fence for BOTH token palettes (issue #180, ADR 0018):
+ * parses tokens.css, so any future palette tweak is re-checked against the
+ * WCAG 2.1 AA thresholds of ADR 0017 — text ≥ 4.5:1, non-text UI ≥ 3:1 —
+ * without anyone having to remember the ratios. Pattern follows
+ * user-color.test.ts.
+ */
+
+const css = readFileSync(new URL('../styles/tokens.css', import.meta.url), 'utf8');
+
+function parseVars(block: string): Record<string, string> {
+  const vars: Record<string, string> = {};
+  for (const match of block.matchAll(/(--color-[\w-]+):\s*([^;]+);/g)) {
+    vars[match[1]!] = match[2]!.trim();
+  }
+  return vars;
+}
+
+function blockFor(selectorStart: string): string {
+  const start = css.indexOf(selectorStart);
+  expect(start, `selector ${selectorStart} present in tokens.css`).toBeGreaterThanOrEqual(0);
+  return css.slice(start, css.indexOf('}', start));
+}
+
+const lightVars = parseVars(blockFor(':root {'));
+const darkVars = { ...lightVars, ...parseVars(blockFor(":root[data-theme='dark']")) };
+
+/** WCAG relative luminance of an sRGB hex colour. */
+function luminance(hex: string): number {
+  expect(hex, `hex colour, got "${hex}"`).toMatch(/^#[0-9a-f]{6}$/i);
+  const channels = [1, 3, 5].map((i) => {
+    const c = parseInt(hex.slice(i, i + 2), 16) / 255;
+    return c <= 0.03928 ? c / 12.92 : ((c + 0.055) / 1.055) ** 2.4;
+  });
+  return 0.2126 * channels[0]! + 0.7152 * channels[1]! + 0.0722 * channels[2]!;
+}
+
+/** Contrast ratio between two colours (WCAG 2.x). */
+function contrast(a: string, b: string): number {
+  const [hi, lo] = [luminance(a), luminance(b)].sort((x, y) => y - x);
+  return (hi! + 0.05) / (lo! + 0.05);
+}
+
+/** [foreground, background, minimum] — only pairings the UI really renders. */
+const PAIRS: [string, string, number][] = [
+  // Body text on every background surface.
+  ['--color-text', '--color-bg', 4.5],
+  ['--color-text', '--color-bg-subtle', 4.5],
+  ['--color-text', '--color-surface', 4.5],
+  ['--color-text', '--color-surface-muted', 4.5],
+  // Muted text (hints, metadata) — not used on surface-muted.
+  ['--color-text-muted', '--color-bg', 4.5],
+  ['--color-text-muted', '--color-bg-subtle', 4.5],
+  ['--color-text-muted', '--color-surface', 4.5],
+  // Accent doubles as link/text colour.
+  ['--color-accent', '--color-bg', 4.5],
+  ['--color-accent', '--color-bg-subtle', 4.5],
+  ['--color-accent', '--color-surface', 4.5],
+  ['--color-accent-contrast', '--color-accent', 4.5],
+  // Danger as text (overdue dates, error notes) and on danger buttons.
+  ['--color-danger', '--color-bg', 4.5],
+  ['--color-danger', '--color-bg-subtle', 4.5],
+  ['--color-danger', '--color-surface', 4.5],
+  ['--color-danger-contrast', '--color-danger', 4.5],
+  ['--color-danger-contrast', '--color-danger-strong', 4.5],
+  // Success text.
+  ['--color-ok', '--color-bg', 4.5],
+  ['--color-ok', '--color-surface', 4.5],
+  // Status badges (admin system page).
+  ['--color-badge-ok-text', '--color-badge-ok-bg', 4.5],
+  ['--color-badge-error-text', '--color-badge-error-bg', 4.5],
+  ['--color-badge-warn-text', '--color-badge-warn-bg', 4.5],
+  // Non-text UI (WCAG 1.4.11): input borders, favorite icons.
+  ['--color-border-input', '--color-bg', 3],
+  ['--color-border-input', '--color-surface', 3],
+  ['--color-favorite', '--color-bg', 3],
+];
+
+describe.each([
+  ['light', lightVars],
+  ['dark', darkVars],
+] as const)('%s palette', (name, vars) => {
+  it.each(PAIRS)('%s on %s clears %s:1', (fg, bg, min) => {
+    const ratio = contrast(vars[fg]!, vars[bg]!);
+    expect(ratio, `${fg} (${vars[fg]}) on ${bg} (${vars[bg]}) in ${name}`).toBeGreaterThanOrEqual(
+      min,
+    );
+  });
+
+  it('defines every token the pair list references', () => {
+    for (const [fg, bg] of PAIRS) {
+      expect(vars[fg], fg).toBeDefined();
+      expect(vars[bg], bg).toBeDefined();
+    }
+  });
+});
diff --git a/apps/web/src/theme/theme.test.ts b/apps/web/src/theme/theme.test.ts
new file mode 100644
index 0000000..e1ae55d
--- /dev/null
+++ b/apps/web/src/theme/theme.test.ts
@@ -0,0 +1,94 @@
+// @vitest-environment jsdom
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { applyTheme, readStoredThemeMode, resolveTheme, THEME_MODE_KEY } from './theme';
+
+/** Node ≥ 22 ships its own (unconfigured, undefined) localStorage global
+ * that shadows jsdom's — give the tests a real in-memory one. */
+function stubLocalStorage(): void {
+  const store = new Map<string, string>();
+  Object.defineProperty(window, 'localStorage', {
+    configurable: true,
+    value: {
+      getItem: (key: string) => store.get(key) ?? null,
+      setItem: (key: string, value: string) => void store.set(key, String(value)),
+      removeItem: (key: string) => void store.delete(key),
+      clear: () => store.clear(),
+    },
+  });
+}
+
+/** jsdom has no matchMedia — stub the single query the module uses. */
+function stubMatchMedia(prefersDark: boolean): void {
+  vi.stubGlobal(
+    'matchMedia',
+    (query: string) =>
+      ({
+        matches: query.includes('dark') && prefersDark,
+        addEventListener: () => undefined,
+      }) as unknown as MediaQueryList,
+  );
+}
+
+beforeEach(() => {
+  stubLocalStorage();
+  stubMatchMedia(false);
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  delete document.documentElement.dataset.theme;
+});
+
+describe('readStoredThemeMode', () => {
+  it('defaults to system when nothing is stored', () => {
+    expect(readStoredThemeMode()).toBe('system');
+  });
+
+  it('reads the JSON-encoded value usePersistentState writes', () => {
+    window.localStorage.setItem(THEME_MODE_KEY, JSON.stringify('dark'));
+    expect(readStoredThemeMode()).toBe('dark');
+  });
+
+  it('falls back to system on broken JSON and unknown values', () => {
+    window.localStorage.setItem(THEME_MODE_KEY, 'not json');
+    expect(readStoredThemeMode()).toBe('system');
+    window.localStorage.setItem(THEME_MODE_KEY, JSON.stringify('purple'));
+    expect(readStoredThemeMode()).toBe('system');
+  });
+});
+
+describe('resolveTheme', () => {
+  it('returns explicit choices untouched', () => {
+    expect(resolveTheme('light')).toBe('light');
+    expect(resolveTheme('dark')).toBe('dark');
+  });
+
+  it('resolves system via prefers-color-scheme', () => {
+    expect(resolveTheme('system')).toBe('light');
+    stubMatchMedia(true);
+    expect(resolveTheme('system')).toBe('dark');
+  });
+});
+
+describe('applyTheme', () => {
+  it('sets a concrete data-theme, color-scheme, and both theme-color metas', () => {
+    for (const media of ['(prefers-color-scheme: light)', '(prefers-color-scheme: dark)']) {
+      const meta = document.createElement('meta');
+      meta.setAttribute('name', 'theme-color');
+      meta.setAttribute('media', media);
+      document.head.appendChild(meta);
+    }
+
+    applyTheme('dark');
+    expect(document.documentElement.dataset.theme).toBe('dark');
+    expect(document.documentElement.style.colorScheme).toBe('dark');
+    const metas = [...document.querySelectorAll('meta[name="theme-color"]')];
+    expect(metas).toHaveLength(2);
+    for (const meta of metas) expect(meta.getAttribute('content')).toBe('#10161d');
+
+    applyTheme('system'); // stubbed OS is light
+    expect(document.documentElement.dataset.theme).toBe('light');
+    for (const meta of metas) expect(meta.getAttribute('content')).toBe('#2f6f4f');
+  });
+});
diff --git a/apps/web/src/theme/theme.ts b/apps/web/src/theme/theme.ts
new file mode 100644
index 0000000..ea3f97a
--- /dev/null
+++ b/apps/web/src/theme/theme.ts
@@ -0,0 +1,84 @@
+import { useEffect, useState } from 'react';
+
+/** localStorage key for the Light/Dark/System choice (issue #180) — a
+ * device-local preference like SINGLE_KEY_SHORTCUTS_KEY, JSON-encoded via
+ * usePersistentState. public/theme-init.js reads the same key before the
+ * bundle loads; keep the two in sync. The ui.theme.* namespace is shared
+ * with the later accent-theming stage (ui.theme.accent, ui.theme.css). */
+export const THEME_MODE_KEY = 'ui.theme.mode';
+
+export type ThemeMode = 'light' | 'dark' | 'system';
+export type EffectiveTheme = 'light' | 'dark';
+
+/** Meta theme-color per effective theme; must match tokens.css
+ * (--color-accent light, --color-bg dark) and the metas in index.html. */
+const THEME_COLOR: Record<EffectiveTheme, string> = {
+  light: '#2f6f4f',
+  dark: '#10161d',
+};
+
+export function readStoredThemeMode(): ThemeMode {
+  try {
+    const raw = window.localStorage.getItem(THEME_MODE_KEY);
+    const parsed = raw === null ? 'system' : (JSON.parse(raw) as unknown);
+    return parsed === 'light' || parsed === 'dark' ? parsed : 'system';
+  } catch {
+    return 'system';
+  }
+}
+
+export function resolveTheme(mode: ThemeMode): EffectiveTheme {
+  if (mode !== 'system') return mode;
+  try {
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+  } catch {
+    return 'light';
+  }
+}
+
+/** Resolve the mode to a CONCRETE theme on <html> — tokens.css keys its
+ * dark palette solely off data-theme (no @media fallback), so this is the
+ * one place the choice takes effect. Also mirrors color-scheme (native
+ * controls) and both media-paired theme-color metas: with an explicit
+ * override the OS scheme must not win, and under 'system' both metas
+ * resolve to the same value anyway. */
+export function applyTheme(mode: ThemeMode): void {
+  const theme = resolveTheme(mode);
+  const root = document.documentElement;
+  root.dataset.theme = theme;
+  root.style.colorScheme = theme;
+  document
+    .querySelectorAll('meta[name="theme-color"]')
+    .forEach((meta) => meta.setAttribute('content', THEME_COLOR[theme]));
+}
+
+/** Follow OS scheme changes live while the stored mode is 'system'.
+ * Called once at startup (main.tsx). */
+export function initSystemThemeListener(): void {
+  try {
+    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', () => {
+      if (readStoredThemeMode() === 'system') applyTheme('system');
+    });
+  } catch {
+    // matchMedia may be absent in exotic embedders; the theme simply stays
+    // as applied until the next explicit change.
+  }
+}
+
+/** The theme currently in effect, reactive to setting AND OS changes —
+ * observes data-theme on <html> instead of duplicating the resolve logic.
+ * Pond-scoped theming (ADR 0018 stage C) will build on this. */
+export function useEffectiveTheme(): EffectiveTheme {
+  const read = (): EffectiveTheme =>
+    document.documentElement.dataset.theme === 'dark' ? 'dark' : 'light';
+  const [theme, setTheme] = useState<EffectiveTheme>(read);
+  useEffect(() => {
+    const observer = new MutationObserver(() => setTheme(read()));
+    observer.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ['data-theme'],
+    });
+    return () => observer.disconnect();
+  }, []);
+  return theme;
+}
diff --git a/docs/architecture/adr/0018-color-theming.md b/docs/architecture/adr/0018-color-theming.md
new file mode 100644
index 0000000..4734250
--- /dev/null
+++ b/docs/architecture/adr/0018-color-theming.md
@@ -0,0 +1,77 @@
+# ADR 0018: Color theming — modes now, accent themes by derivation later
+
+- Status: accepted
+- Date: 2026-07-28
+
+## Context
+
+Until issue #180 the SPA was light-only; the no-JS public shell carried a
+minimal dark block as a contrast fix (#167). ADR 0016 established the
+token pattern for fonts (`--font-*` slots, pond-scoped overrides via
+custom properties); ADR 0017 makes WCAG 2.1 AA contrast a hard
+requirement. The product direction is a staged theming vision: dark mode
+first, then user-selectable accent themes, then pond-level accents.
+
+This ADR fixes the model for the whole roadmap so the later stages do not
+need new architecture decisions.
+
+## Decision
+
+**Model: theme = mode × accent.** The _mode_ (light/dark) selects one of
+two complete neutral palettes; the _accent_ (later stages) recolors only
+the accent token family. The two axes are orthogonal.
+
+1. **One token palette per mode, in `tokens.css` only.** Light values
+   live on `:root`, dark values in a single `:root[data-theme='dark']`
+   block. There is deliberately **no `@media` duplicate**: JS always
+   resolves the user's Light/Dark/System choice to a _concrete_
+   `data-theme` on `<html>` (`theme/theme.ts`; System via `matchMedia`
+   plus a live change listener). This single selector is the stable
+   contract later stages mirror.
+2. **Every color is a token pair.** New colors MUST be added to both
+   blocks (or deliberately inherit) and their real UI pairings MUST pass
+   the mechanical contrast fence `theme-contrast.test.ts` (text ≥ 4.5:1,
+   non-text UI ≥ 3:1). `color-scheme` flips with the palette so native
+   controls and scrollbars follow.
+3. **The mode preference is device-local.** `ui.theme.mode` in
+   localStorage (like the single-key-shortcut toggle, #170) — never
+   server-persisted. The `ui.theme.*` namespace is reserved for the
+   accent stage (`ui.theme.accent`, `ui.theme.css` pre-derived CSS
+   cache).
+4. **Flash prevention is a classic external script.** The prod CSP
+   (`script-src 'self'`) forbids inline scripts, so
+   `public/theme-init.js` runs during `<head>` parsing, before first
+   paint and the module bundle, and already injects the (future)
+   `ui.theme.css` cache. A parse-time `<meta name="color-scheme">` plus
+   paired `theme-color` metas cover the interval before it runs;
+   `applyTheme()` rewrites both metas so an explicit override beats the
+   OS scheme.
+5. **Accent stages derive, they do not validate.** A future "color
+   theme" is ONE accent hex (presets = curated hexes, custom = free
+   pick); `deriveAccentTokens(hex, mode)` computes accent tokens whose
+   contrast is guaranteed _by construction_ against the canonical mode
+   backgrounds. Full-palette themes (recoloring neutrals) are explicitly
+   out of scope: the contrast surface explodes, and plugin CSS
+   (`section-styles-basic`) relies on near-neutral backgrounds without
+   knowing host tokens. Pond theming recolors the accent family only, as
+   a scoped wrapper like `PondFontScope` (ADR 0016), with precedence
+   pond > user > default via the natural custom-property cascade;
+   `useEffectiveTheme()` exists for that stage.
+6. **The no-JS public shell stays self-contained.** `html-shell.ts`
+   keeps its own inline light/dark CSS via `@media` (no JS there) with
+   colors matching the SPA dark tokens; it is deliberately not
+   user/pond-themed for now.
+
+## Consequences
+
+- Adding a color means adding a _pair_ plus a fence entry — forgetting
+  the dark value or an inadequate ratio fails unit tests, not review.
+- The a11y e2e pack scans every core screen in both schemes; axe measures
+  real computed dark colors.
+- Accepted trade-offs: `USER_COLORS` (presence/carets) stay
+  theme-independent — their accessible signal is the white text on them;
+  the PWA manifest splash stays light (manifests cannot media-query);
+  the mode preference does not roam across devices.
+- `--color-primary` no longer exists; the accent family is
+  `--color-accent`/`--color-accent-contrast` (plus the danger family for
+  destructive surfaces).
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 7e1782e..8968a3f 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -36,6 +36,17 @@ export default tseslint.config(
       },
     },
   },
+  {
+    // Classic browser scripts served verbatim from public/ (theme-init.js,
+    // issue #180): no bundler, no modules, browser globals only.
+    files: ['apps/web/public/*.js'],
+    languageOptions: {
+      globals: {
+        window: 'readonly',
+        document: 'readonly',
+      },
+    },
+  },
   {
     rules: {
       // Unused values are usually bugs; underscore-prefix marks intentional ones.
diff --git a/packages/shared/i18n/de/settings.json b/packages/shared/i18n/de/settings.json
index bc06e69..f9ec6a0 100644
--- a/packages/shared/i18n/de/settings.json
+++ b/packages/shared/i18n/de/settings.json
@@ -57,6 +57,14 @@
     "save": "Startseite speichern",
     "saved": "Gespeichert."
   },
+  "appearance": {
+    "title": "Erscheinungsbild",
+    "legend": "Farbschema",
+    "light": "Hell",
+    "dark": "Dunkel",
+    "system": "Systemeinstellung",
+    "hint": "„Systemeinstellung“ folgt dem Hell-/Dunkel-Modus des Geräts. Gilt für dieses Gerät."
+  },
   "interaction": {
     "title": "Bedienung",
     "disableSingleKey": "Einzeltasten-Kürzel deaktivieren",
diff --git a/packages/shared/i18n/en/settings.json b/packages/shared/i18n/en/settings.json
index 9d208dc..f34d578 100644
--- a/packages/shared/i18n/en/settings.json
+++ b/packages/shared/i18n/en/settings.json
@@ -57,6 +57,14 @@
     "save": "Save landing page",
     "saved": "Saved."
   },
+  "appearance": {
+    "title": "Appearance",
+    "legend": "Color scheme",
+    "light": "Light",
+    "dark": "Dark",
+    "system": "System setting",
+    "hint": "“System setting” follows the device’s light/dark mode. Applies to this device."
+  },
   "interaction": {
     "title": "Interaction",
     "disableSingleKey": "Disable single-key shortcuts",