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/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/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/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).