From b5d2a436e0d526c01a3e1cc85594f8b60ca904d6 Mon Sep 17 00:00:00 2001
From: Claude Fable 5 <fable-5@waidele.info>
Date: Wed, 29 Jul 2026 09:09:36 +0200
Subject: [PATCH] =?UTF-8?q?#186:=20pond=20accent=20theming=20=E2=80=94=20s?=
 =?UTF-8?q?coped=20derivation,=20cascade=20pond=20>=20user=20>=20default?=
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Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

pondSettingsSchema gains theme = { accent: '#rrggbb' | null } (null =
inherit the viewer's theme), exposed as a top-level key of the flat
updatePondInputSchema and included in the PondsService settings merge
(the known silent-no-op pitfall). The server validates only the hex;
conformance arises at render time: PondThemeScope (mounted around the
page content next to PondFontScope) derives the accent pair for the
EFFECTIVE mode via useEffectiveTheme and sets it as inline custom
properties — inline beats both tokens.css and the user-theme <style>,
which IS the cascade precedence pond > user > default.

Pond settings get a PondThemeSection (inherit | presets | custom color
with per-mode preview swatches, explicit save like the font manager);
AccentSwatches extracted for reuse; i18n de+en. The no-JS public shell
stays deliberately un-themed (ADR 0018 amendment).

Tests: pond DB test (theme merge keeps fonts, invalid hex 400), e2e
pond-theme.spec (scope boundary content vs. chrome, per-mode
re-derivation, axe on the pond settings page; resets the fixture pond).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QRtCnB3uLdQtFmvp9HXcRX
---
 apps/api/src/ponds/ponds.e2e.db.test.ts     |  38 +++++
 apps/api/src/ponds/ponds.service.ts         |   4 +-
 apps/web/e2e/pond-theme.spec.ts             |  72 +++++++++
 apps/web/src/pages/PageEditorPage.tsx       | 157 ++++++++++----------
 apps/web/src/pages/PondSettingsPage.tsx     |   6 +
 apps/web/src/pages/SettingsPage.tsx         |  20 +--
 apps/web/src/theme/AccentSwatches.tsx       |  25 ++++
 apps/web/src/theme/PondThemeScope.tsx       |  40 +++++
 apps/web/src/theme/PondThemeSection.tsx     | 122 +++++++++++++++
 docs/architecture/adr/0018-color-theming.md |  17 +++
 packages/shared/i18n/de/font.json           |   7 +
 packages/shared/i18n/en/font.json           |   7 +
 packages/shared/src/ponds.ts                |  17 +++
 13 files changed, 435 insertions(+), 97 deletions(-)
 create mode 100644 apps/web/e2e/pond-theme.spec.ts
 create mode 100644 apps/web/src/theme/AccentSwatches.tsx
 create mode 100644 apps/web/src/theme/PondThemeScope.tsx
 create mode 100644 apps/web/src/theme/PondThemeSection.tsx

diff --git a/apps/api/src/ponds/ponds.e2e.db.test.ts b/apps/api/src/ponds/ponds.e2e.db.test.ts
index 1fc8005..390b305 100644
--- a/apps/api/src/ponds/ponds.e2e.db.test.ts
+++ b/apps/api/src/ponds/ponds.e2e.db.test.ts
@@ -146,6 +146,44 @@ describe.skipIf(!hasTestDb)('ponds (e2e, issue #21)', () => {
     expect(fetched.body.name).toBe(`Neuer Name ${suffix}`);
   });
 
+  it('saves the pond accent theme without losing other settings (issue #186)', async () => {
+    const created = await api()
+      .post('/api/v1/ponds')
+      .set('Cookie', ownerCookie)
+      .send({ name: `Akzent ${suffix}` })
+      .expect(201);
+
+    // First persist a non-default font, then the theme — the settings
+    // merge must keep both (the jsonb stores only deviations).
+    await api()
+      .patch(`/api/v1/ponds/${created.body.id}`)
+      .set('Cookie', ownerCookie)
+      .send({
+        fonts: { ...created.body.settings.fonts, heading: { family: 'Roboto', weight: 700 } },
+      })
+      .expect(200);
+    const themed = await api()
+      .patch(`/api/v1/ponds/${created.body.id}`)
+      .set('Cookie', ownerCookie)
+      .send({ theme: { accent: '#2b5f8f' } })
+      .expect(200);
+    expect(themed.body.settings.theme.accent).toBe('#2b5f8f');
+    expect(themed.body.settings.fonts.heading.weight).toBe(700);
+
+    // Back to inherit; malformed hexes never reach the settings.
+    const inherit = await api()
+      .patch(`/api/v1/ponds/${created.body.id}`)
+      .set('Cookie', ownerCookie)
+      .send({ theme: { accent: null } })
+      .expect(200);
+    expect(inherit.body.settings.theme.accent).toBeNull();
+    await api()
+      .patch(`/api/v1/ponds/${created.body.id}`)
+      .set('Cookie', ownerCookie)
+      .send({ theme: { accent: 'lila' } })
+      .expect(400);
+  });
+
   it('hides foreign ponds (list and slug lookup)', async () => {
     const created = await api()
       .post('/api/v1/ponds')
diff --git a/apps/api/src/ponds/ponds.service.ts b/apps/api/src/ponds/ponds.service.ts
index 60384cf..c379409 100644
--- a/apps/api/src/ponds/ponds.service.ts
+++ b/apps/api/src/ponds/ponds.service.ts
@@ -154,7 +154,8 @@ export class PondsService {
       input.fonts !== undefined ||
       input.commentPolicy !== undefined ||
       input.apiEnabled !== undefined ||
-      input.mcpEnabled !== undefined;
+      input.mcpEnabled !== undefined ||
+      input.theme !== undefined;
     const settings = !settingsChanged
       ? undefined
       : {
@@ -165,6 +166,7 @@ export class PondsService {
           ...(input.commentPolicy !== undefined ? { commentPolicy: input.commentPolicy } : {}),
           ...(input.apiEnabled !== undefined ? { apiEnabled: input.apiEnabled } : {}),
           ...(input.mcpEnabled !== undefined ? { mcpEnabled: input.mcpEnabled } : {}),
+          ...(input.theme !== undefined ? { theme: input.theme } : {}),
         };
     const updated = await this.prisma.pond.update({
       where: { id },
diff --git a/apps/web/e2e/pond-theme.spec.ts b/apps/web/e2e/pond-theme.spec.ts
new file mode 100644
index 0000000..14864be
--- /dev/null
+++ b/apps/web/e2e/pond-theme.spec.ts
@@ -0,0 +1,72 @@
+import AxeBuilder from '@axe-core/playwright';
+import { expect, test, type Page } from '@playwright/test';
+
+import { contextForUser } from './helpers';
+
+/**
+ * Teich-Akzent (issue #186, ADR 0018 Stufe C): der Akzent eines Teichs
+ * färbt NUR die Teich-Inhalte (PondThemeScope), das App-Chrome behält den
+ * Akzent der Betrachtenden; je Modus wird abgeleitet. Der Spec setzt das
+ * Theme des content-fixtures-Teichs per API und RÄUMT ES AM ENDE WIEDER AB
+ * (Fixtures sind pack-übergreifend geteilt).
+ */
+
+const BASE = process.env.E2E_BASE_URL ?? 'http://localhost:5173';
+const POND = 'content-fixtures';
+const ACCENT = '#6d4fa0'; // iris-violet: besteht hell verbatim, dunkel wird abgeleitet
+
+const accentAt = (page: Page, selector: string) =>
+  page.evaluate(
+    (sel) =>
+      getComputedStyle(document.querySelector(sel)!).getPropertyValue('--color-accent').trim(),
+    selector,
+  );
+
+test('pond accent colors the pond content but not the app chrome', async ({ browser }) => {
+  const context = await contextForUser(browser, BASE, 'fixture-user');
+  const page = await context.newPage();
+  await page.emulateMedia({ colorScheme: 'light' });
+
+  const pond = await page.request.get(`${BASE}/api/v1/ponds/${POND}`);
+  expect(pond.ok()).toBe(true);
+  const pondId = ((await pond.json()) as { id: string }).id;
+
+  try {
+    const patch = await page.request.patch(`${BASE}/api/v1/ponds/${pondId}`, {
+      data: { theme: { accent: ACCENT } },
+    });
+    expect(patch.ok()).toBe(true);
+
+    await page.goto(`/p/${POND}/every-element`);
+    await page.locator('.pond-theme-scope').waitFor();
+
+    // Scope-Grenze: Inhalt trägt den Teich-Akzent, die TopBar den Default.
+    expect(await accentAt(page, '.pond-theme-scope')).toBe(ACCENT);
+    expect(await accentAt(page, '.topbar')).toBe('#2f6f4f');
+
+    // Moduswechsel leitet neu ab: dunkel bekommt einen helleren Wert.
+    await page.locator('.topbar__theme').click(); // system -> light
+    await page.locator('.topbar__theme').click(); // light -> dark
+    const darkAccent = await accentAt(page, '.pond-theme-scope');
+    expect(darkAccent).not.toBe(ACCENT);
+    expect(darkAccent).toMatch(/^#[0-9a-f]{6}$/);
+    expect(await accentAt(page, '.topbar')).not.toBe(ACCENT);
+
+    // Die Teich-Einstellungen (mit neuer Akzent-Sektion) bleiben axe-sauber.
+    await page.goto(`/p/${POND}/settings`);
+    await page.locator('input[name="pond-theme-accent"]').first().waitFor();
+    const results = await new AxeBuilder({ page })
+      .withTags(['wcag2a', 'wcag21a', 'wcag2aa', 'wcag21aa'])
+      .analyze();
+    expect(
+      results.violations.map((v) => ({ rule: v.id, help: v.help })),
+      'axe auf den Teich-Einstellungen',
+    ).toEqual([]);
+  } finally {
+    await page.request.patch(`${BASE}/api/v1/ponds/${pondId}`, {
+      data: { theme: { accent: null } },
+    });
+  }
+
+  await context.close();
+});
diff --git a/apps/web/src/pages/PageEditorPage.tsx b/apps/web/src/pages/PageEditorPage.tsx
index 85fde37..bbc3d76 100644
--- a/apps/web/src/pages/PageEditorPage.tsx
+++ b/apps/web/src/pages/PageEditorPage.tsx
@@ -23,6 +23,7 @@ import { BacklinksPanel } from '../links/BacklinksPanel';
 import { collaborationCaretFor } from '../editor/collaboration-caret';
 import { documentExtensions } from '../editor/document-extensions';
 import { PondFontScope } from '../fonts/PondFontScope';
+import { PondThemeScope } from '../theme/PondThemeScope';
 import { ImageUpload } from '../editor/image-upload';
 import { PresenceStrip } from '../editor/PresenceStrip';
 import { Toolbar } from '../editor/Toolbar';
@@ -538,85 +539,87 @@ export function PageEditorPage(): React.JSX.Element {
   }
 
   return (
-    <PondFontScope fonts={pond.data?.settings.fonts ?? DEFAULT_POND_FONTS}>
-      {/* The page's actions render as icons in the TopBar (issue #101). */}
-      {actionsSlot.element &&
-        createPortal(
-          <PageActions
-            pageId={resolved.id}
-            slug={resolved.slug}
-            pondSlug={pondSlug}
-            mode={mode}
-            onToggleMode={() => setMode(mode === 'edit' ? 'view' : 'edit')}
-            showAttachments={showAttachments}
-            onToggleAttachments={() => setShowAttachments((open) => !open)}
-            hasTools={hasPageTools(pagePlugins.data)}
-            showPageTools={showPageTools}
-            onTogglePageTools={() => setShowPageTools((open) => !open)}
-            showLabels={showLabels}
-            onToggleLabels={() => setShowLabels((open) => !open)}
-            showHistory={showHistory}
-            onToggleHistory={() => setShowHistory((open) => !open)}
-          />,
-          actionsSlot.element,
-        )}
-      <div className="editor-page">
-        <div className="editor-page__header">
-          {/* The visible title is an input; give assistive tech the page
-              heading it expects on an article view (#166). */}
-          <h1 className="visually-hidden">{title || t('title.placeholder')}</h1>
-          <input
-            type="text"
-            className="editor-page__title"
-            value={title}
-            placeholder={t('title.placeholder')}
-            aria-label={t('title.label')}
-            disabled={mode !== 'edit'}
-            onChange={(event) => setTitle(event.target.value)}
-            onBlur={() => void saveTitle()}
-          />
-        </div>
-        {/* Status line between the header and the article (#134): last update,
-            word count, reading time — reading mode only. */}
-        {mode === 'view' && page.data && (
-          <PageStatusBar updatedAt={page.data.updatedAt} wordCount={wordCount} />
-        )}
-        <div className="editor-page__body">
-          <PageEditor
-            page={resolved}
-            mode={mode}
-            pondSlug={pondSlug}
-            showAttachments={showAttachments}
-            showPageTools={showPageTools}
-            onCloseAttachments={() => setShowAttachments(false)}
-            onWriteAccess={setCanWrite}
-          />
-          {/* Side panels stack vertically in one column (M10 follow-up). */}
-          {(showLabels || showHistory) && (
-            <div className="editor-page__panels">
-              {showLabels && (
-                <LabelPicker
-                  pageId={resolved.id}
-                  pondId={resolved.pondId}
-                  pondSlug={pondSlug}
-                  onClose={() => setShowLabels(false)}
-                />
-              )}
-              {showHistory && (
-                <HistoryPanel pageId={resolved.id} onClose={() => setShowHistory(false)} />
-              )}
-            </div>
+    <PondThemeScope theme={pond.data?.settings.theme ?? { accent: null }}>
+      <PondFontScope fonts={pond.data?.settings.fonts ?? DEFAULT_POND_FONTS}>
+        {/* The page's actions render as icons in the TopBar (issue #101). */}
+        {actionsSlot.element &&
+          createPortal(
+            <PageActions
+              pageId={resolved.id}
+              slug={resolved.slug}
+              pondSlug={pondSlug}
+              mode={mode}
+              onToggleMode={() => setMode(mode === 'edit' ? 'view' : 'edit')}
+              showAttachments={showAttachments}
+              onToggleAttachments={() => setShowAttachments((open) => !open)}
+              hasTools={hasPageTools(pagePlugins.data)}
+              showPageTools={showPageTools}
+              onTogglePageTools={() => setShowPageTools((open) => !open)}
+              showLabels={showLabels}
+              onToggleLabels={() => setShowLabels((open) => !open)}
+              showHistory={showHistory}
+              onToggleHistory={() => setShowHistory((open) => !open)}
+            />,
+            actionsSlot.element,
           )}
-        </div>
-        {/* "Linked from" appears below the content in read mode (issue #48);
+        <div className="editor-page">
+          <div className="editor-page__header">
+            {/* The visible title is an input; give assistive tech the page
+              heading it expects on an article view (#166). */}
+            <h1 className="visually-hidden">{title || t('title.placeholder')}</h1>
+            <input
+              type="text"
+              className="editor-page__title"
+              value={title}
+              placeholder={t('title.placeholder')}
+              aria-label={t('title.label')}
+              disabled={mode !== 'edit'}
+              onChange={(event) => setTitle(event.target.value)}
+              onBlur={() => void saveTitle()}
+            />
+          </div>
+          {/* Status line between the header and the article (#134): last update,
+            word count, reading time — reading mode only. */}
+          {mode === 'view' && page.data && (
+            <PageStatusBar updatedAt={page.data.updatedAt} wordCount={wordCount} />
+          )}
+          <div className="editor-page__body">
+            <PageEditor
+              page={resolved}
+              mode={mode}
+              pondSlug={pondSlug}
+              showAttachments={showAttachments}
+              showPageTools={showPageTools}
+              onCloseAttachments={() => setShowAttachments(false)}
+              onWriteAccess={setCanWrite}
+            />
+            {/* Side panels stack vertically in one column (M10 follow-up). */}
+            {(showLabels || showHistory) && (
+              <div className="editor-page__panels">
+                {showLabels && (
+                  <LabelPicker
+                    pageId={resolved.id}
+                    pondId={resolved.pondId}
+                    pondSlug={pondSlug}
+                    onClose={() => setShowLabels(false)}
+                  />
+                )}
+                {showHistory && (
+                  <HistoryPanel pageId={resolved.id} onClose={() => setShowHistory(false)} />
+                )}
+              </div>
+            )}
+          </div>
+          {/* "Linked from" appears below the content in read mode (issue #48);
             the inline discussion (issue #133) and the local neighborhood graph
             (issue #113) follow it, in that order. */}
-        {mode === 'view' && <BacklinksPanel pageId={resolved.id} pondSlug={pondSlug} />}
-        {mode === 'view' && <CommentsSection pageId={resolved.id} mayComment={mayComment} />}
-        {mode === 'view' && (
-          <LocalGraphPanel pageId={resolved.id} pondId={resolved.pondId} pondSlug={pondSlug} />
-        )}
-      </div>
-    </PondFontScope>
+          {mode === 'view' && <BacklinksPanel pageId={resolved.id} pondSlug={pondSlug} />}
+          {mode === 'view' && <CommentsSection pageId={resolved.id} mayComment={mayComment} />}
+          {mode === 'view' && (
+            <LocalGraphPanel pageId={resolved.id} pondId={resolved.pondId} pondSlug={pondSlug} />
+          )}
+        </div>
+      </PondFontScope>
+    </PondThemeScope>
   );
 }
diff --git a/apps/web/src/pages/PondSettingsPage.tsx b/apps/web/src/pages/PondSettingsPage.tsx
index 43f7c3f..91e6361 100644
--- a/apps/web/src/pages/PondSettingsPage.tsx
+++ b/apps/web/src/pages/PondSettingsPage.tsx
@@ -21,6 +21,7 @@ import { SidebarViewSetting } from '../layout/SidebarViewSetting';
 import { MemberManager } from '../members/MemberManager';
 import { DeletePondSection } from '../ponds/DeletePondSection';
 import { PondPluginSettings } from '../plugins/PondPluginSettings';
+import { PondThemeSection } from '../theme/PondThemeSection';
 
 import { useDocumentTitle } from '../lib/use-document-title';
 /**
@@ -106,6 +107,11 @@ export function PondSettingsPage(): React.JSX.Element {
               pondSlug={pondSlug}
               fonts={pond.data.settings.fonts}
             />
+            <PondThemeSection
+              pondId={pond.data.id}
+              pondSlug={pondSlug}
+              theme={pond.data.settings.theme}
+            />
           </section>
         )}
         {canModify && <PondPluginSettings pondId={pond.data.id} />}
diff --git a/apps/web/src/pages/SettingsPage.tsx b/apps/web/src/pages/SettingsPage.tsx
index 6a29ddb..229ce07 100644
--- a/apps/web/src/pages/SettingsPage.tsx
+++ b/apps/web/src/pages/SettingsPage.tsx
@@ -1,6 +1,5 @@
 import { zodResolver } from '@hookform/resolvers/zod';
 import {
-  BASE_PALETTE,
   changePasswordInputSchema,
   DEFAULT_THEME_PRESET_ID,
   deriveAccentTokens,
@@ -24,6 +23,7 @@ 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 { AccentSwatches } from '../theme/AccentSwatches';
 import { useAccentChoice } from '../theme/apply-theme';
 import { useThemeMode, type ThemeMode } from '../theme/theme';
 interface SessionView {
@@ -91,24 +91,6 @@ function AppearanceSection(): React.JSX.Element {
   );
 }
 
-/** Per-mode preview of a derived accent. Decorative only — the visible
- * preset/custom NAME carries the meaning, color is never the only cue
- * (ADR 0017). The rings show each mode's canonical background. */
-function AccentSwatches({ light, dark }: { light: string; dark: string }): React.JSX.Element {
-  return (
-    <span className="accent-swatches" aria-hidden>
-      <span
-        className="accent-swatch"
-        style={{ background: light, boxShadow: `0 0 0 3px ${BASE_PALETTE.light.bg}` }}
-      />
-      <span
-        className="accent-swatch"
-        style={{ background: dark, boxShadow: `0 0 0 3px ${BASE_PALETTE.dark.bg}` }}
-      />
-    </span>
-  );
-}
-
 /** Akzentfarbe (issue #184, ADR 0018 stage B): presets and a free color as
  * ONE mechanism — both run through deriveAccentTokens, so any pick stays
  * readable by construction. Same section as the mode radios (the jump-nav
diff --git a/apps/web/src/theme/AccentSwatches.tsx b/apps/web/src/theme/AccentSwatches.tsx
new file mode 100644
index 0000000..7371a22
--- /dev/null
+++ b/apps/web/src/theme/AccentSwatches.tsx
@@ -0,0 +1,25 @@
+import { BASE_PALETTE } from '@dorfteich/shared';
+
+/** Per-mode preview of a derived accent (issues #184/#186). Decorative only
+ * — the visible preset/custom NAME carries the meaning, color is never the
+ * only cue (ADR 0017). The rings show each mode's canonical background. */
+export function AccentSwatches({
+  light,
+  dark,
+}: {
+  light: string;
+  dark: string;
+}): React.JSX.Element {
+  return (
+    <span className="accent-swatches" aria-hidden>
+      <span
+        className="accent-swatch"
+        style={{ background: light, boxShadow: `0 0 0 3px ${BASE_PALETTE.light.bg}` }}
+      />
+      <span
+        className="accent-swatch"
+        style={{ background: dark, boxShadow: `0 0 0 3px ${BASE_PALETTE.dark.bg}` }}
+      />
+    </span>
+  );
+}
diff --git a/apps/web/src/theme/PondThemeScope.tsx b/apps/web/src/theme/PondThemeScope.tsx
new file mode 100644
index 0000000..ebaacdb
--- /dev/null
+++ b/apps/web/src/theme/PondThemeScope.tsx
@@ -0,0 +1,40 @@
+import { deriveAccentTokens, type PondTheme } from '@dorfteich/shared';
+import type { CSSProperties, ReactNode } from 'react';
+
+import { useEffectiveTheme, type EffectiveTheme } from './theme';
+
+/**
+ * The custom properties a pond's accent sets on its content root (issue
+ * #186, ADR 0018 stage C) — derived per EFFECTIVE mode with the same engine
+ * as the user accent (#184). Empty when the pond inherits (`accent: null`).
+ */
+export function pondThemeVariables(theme: PondTheme, mode: EffectiveTheme): CSSProperties {
+  if (theme.accent === null) return {};
+  const tokens = deriveAccentTokens(theme.accent, mode);
+  return {
+    '--color-accent': tokens.accent,
+    '--color-accent-contrast': tokens.accentContrast,
+  } as CSSProperties;
+}
+
+/**
+ * Applies a pond's accent to its content, analog PondFontScope (ADR 0016):
+ * the INLINE custom properties win over both the tokens.css palette and the
+ * user's `<style id="user-theme">` override inside this subtree — that IS
+ * the cascade precedence pond > user > default. Re-renders on mode flips
+ * via useEffectiveTheme (data-theme observer).
+ */
+export function PondThemeScope({
+  theme,
+  children,
+}: {
+  theme: PondTheme;
+  children: ReactNode;
+}): React.JSX.Element {
+  const mode = useEffectiveTheme();
+  return (
+    <div className="pond-theme-scope" style={pondThemeVariables(theme, mode)}>
+      {children}
+    </div>
+  );
+}
diff --git a/apps/web/src/theme/PondThemeSection.tsx b/apps/web/src/theme/PondThemeSection.tsx
new file mode 100644
index 0000000..c00d249
--- /dev/null
+++ b/apps/web/src/theme/PondThemeSection.tsx
@@ -0,0 +1,122 @@
+import { deriveAccentTokens, THEME_PRESETS, type PondTheme } from '@dorfteich/shared';
+import { useQueryClient } from '@tanstack/react-query';
+import { useState } from 'react';
+import { useTranslation } from 'react-i18next';
+
+import { FormError } from '../components/forms';
+import { apiPatch } from '../lib/api';
+import { AccentSwatches } from './AccentSwatches';
+
+/**
+ * Pond accent setting (issue #186, ADR 0018 stage C): inherit | preset |
+ * custom color, applied to the pond's content via PondThemeScope. Explicit
+ * save like the font manager (AppearanceManager); Pond-Admin-gated by the
+ * api's PATCH guard. Presets here include the default green — unlike the
+ * user setting, picking it PINS the pond to green for every viewer.
+ */
+export function PondThemeSection({
+  pondId,
+  pondSlug,
+  theme,
+}: {
+  pondId: string;
+  pondSlug: string;
+  theme: PondTheme;
+}): React.JSX.Element {
+  const { t } = useTranslation('font');
+  const { t: tSettings } = useTranslation('settings');
+  const queryClient = useQueryClient();
+  const [accent, setAccent] = useState<string | null>(theme.accent);
+  const [status, setStatus] = useState<'idle' | 'saving' | 'saved'>('idle');
+  const [error, setError] = useState<unknown>(null);
+
+  const presetHexes = THEME_PRESETS.map((preset) => preset.accent);
+  const isCustom = accent !== null && !presetHexes.includes(accent);
+  const [customHex, setCustomHex] = useState(isCustom ? accent : '#2f6f4f');
+
+  const choose = (value: string | null): void => {
+    setAccent(value);
+    setStatus('idle');
+  };
+
+  const derivedPair = (hex: string): { light: string; dark: string } => ({
+    light: deriveAccentTokens(hex, 'light').accent,
+    dark: deriveAccentTokens(hex, 'dark').accent,
+  });
+
+  async function save(): Promise<void> {
+    setStatus('saving');
+    setError(null);
+    try {
+      await apiPatch(`/ponds/${pondId}`, { theme: { accent } });
+      await queryClient.invalidateQueries({ queryKey: ['pond', pondSlug] });
+      setStatus('saved');
+    } catch (err) {
+      setError(err);
+      setStatus('idle');
+    }
+  }
+
+  return (
+    <div className="pond-theme">
+      <FormError error={error} />
+      <fieldset className="settings-fieldset">
+        <legend>{t('pondTheme.legend')}</legend>
+        <label className="settings-checkbox">
+          <input
+            type="radio"
+            name="pond-theme-accent"
+            value=""
+            checked={accent === null}
+            onChange={() => choose(null)}
+          />
+          {t('pondTheme.inherit')}
+        </label>
+        {THEME_PRESETS.map((preset) => (
+          <label key={preset.id} className="settings-checkbox">
+            <input
+              type="radio"
+              name="pond-theme-accent"
+              value={preset.accent}
+              checked={accent === preset.accent}
+              onChange={() => choose(preset.accent)}
+            />
+            {tSettings(`appearance.presets.${preset.id}`)}
+            <AccentSwatches {...derivedPair(preset.accent)} />
+          </label>
+        ))}
+        <div className="settings-checkbox">
+          <input
+            type="radio"
+            id="pond-theme-custom"
+            name="pond-theme-accent"
+            value="custom"
+            checked={isCustom}
+            onChange={() => choose(customHex)}
+          />
+          <label htmlFor="pond-theme-custom">{tSettings('appearance.custom')}</label>
+          <input
+            type="color"
+            className="accent-color-input"
+            value={isCustom ? accent : customHex}
+            aria-label={tSettings('appearance.customPick')}
+            onChange={(event) => {
+              setCustomHex(event.target.value);
+              choose(event.target.value);
+            }}
+          />
+          {isCustom && <AccentSwatches {...derivedPair(accent)} />}
+        </div>
+      </fieldset>
+      <p className="field__hint">{t('pondTheme.hint')}</p>
+      <button
+        type="button"
+        className="button"
+        onClick={() => void save()}
+        disabled={status === 'saving'}
+      >
+        {status === 'saved' ? t('pondTheme.saved') : t('pondTheme.save')}
+      </button>
+    </div>
+  );
+}
diff --git a/docs/architecture/adr/0018-color-theming.md b/docs/architecture/adr/0018-color-theming.md
index 2fb33de..b4eb62b 100644
--- a/docs/architecture/adr/0018-color-theming.md
+++ b/docs/architecture/adr/0018-color-theming.md
@@ -103,3 +103,20 @@ The derivation from decision 5 is implemented as dependency-free
   the pre-derived CSS cache `ui.theme.css`, which `theme-init.js`
   injects before first paint; `main.tsx` re-derives from the CHOICE at
   startup so app updates heal stale caches.
+
+## Amendment: stage C — pond accent shipped (issue #186)
+
+- `pondSettingsSchema.theme = { accent: '#rrggbb' | null }` (`null` =
+  inherit the viewer's theme), also a top-level key of the flat
+  `updatePondInputSchema` and of the PondsService settings merge. The
+  server validates only the hex — conformance arises at render time via
+  the same `deriveAccentTokens`, per mode.
+- `PondThemeScope` (mounted around the page content next to
+  `PondFontScope`) sets the accent pair as INLINE custom properties
+  derived for the effective mode (`useEffectiveTheme`). Inline wins over
+  both tokens.css and the user's `<style id="user-theme">` — that IS the
+  cascade precedence pond > user > default; no extra machinery.
+- Only the accent family is pond-themable (decision 5's boundary), and
+  the no-JS public shell stays deliberately un-themed in v1 — the shared
+  engine leaves that door open (server-side derivation would be a small
+  amendment, not a new decision).
diff --git a/packages/shared/i18n/de/font.json b/packages/shared/i18n/de/font.json
index 7051e3b..6d403b3 100644
--- a/packages/shared/i18n/de/font.json
+++ b/packages/shared/i18n/de/font.json
@@ -24,5 +24,12 @@
     "sans-serif": "Serifenlos",
     "serif": "Serif",
     "monospace": "Dicktengleich"
+  },
+  "pondTheme": {
+    "legend": "Akzentfarbe des Teichs",
+    "inherit": "Eigene Einstellung der Betrachtenden (Standard)",
+    "hint": "Färbt Links, Buttons und Hervorhebungen der Teich-Inhalte für alle Betrachtenden. Die Farbe wird je Hell-/Dunkelmodus automatisch angepasst, damit Texte lesbar bleiben.",
+    "save": "Akzentfarbe speichern",
+    "saved": "Gespeichert"
   }
 }
diff --git a/packages/shared/i18n/en/font.json b/packages/shared/i18n/en/font.json
index fe4193a..89468c8 100644
--- a/packages/shared/i18n/en/font.json
+++ b/packages/shared/i18n/en/font.json
@@ -24,5 +24,12 @@
     "sans-serif": "Sans-serif",
     "serif": "Serif",
     "monospace": "Monospace"
+  },
+  "pondTheme": {
+    "legend": "Pond accent color",
+    "inherit": "Each viewer's own setting (default)",
+    "hint": "Colors links, buttons, and highlights of this pond's content for every viewer. The color is automatically adjusted per light/dark mode so text stays readable.",
+    "save": "Save accent color",
+    "saved": "Saved"
   }
 }
diff --git a/packages/shared/src/ponds.ts b/packages/shared/src/ponds.ts
index 5328213..295a4ac 100644
--- a/packages/shared/src/ponds.ts
+++ b/packages/shared/src/ponds.ts
@@ -39,6 +39,21 @@ export type PondFonts = z.infer<typeof pondFontsSchema>;
  * ADR 0016) — persisted settings therefore only need to store what the
  * pond actually changed.
  */
+/**
+ * Pond accent theme (issue #186, ADR 0018 stage C): `null` inherits the
+ * viewer's own theme. Only the accent family is pond-themable; the hex is
+ * validated here, WCAG conformance arises at render time via
+ * `deriveAccentTokens` — never as a server-side check.
+ */
+export const pondThemeSchema = z.object({
+  accent: z
+    .string()
+    .regex(/^#[0-9a-f]{6}$/i, 'validation.invalid')
+    .nullable()
+    .default(null),
+});
+export type PondTheme = z.infer<typeof pondThemeSchema>;
+
 export const pondSettingsSchema = z.object({
   sidebarSort: z.enum(SIDEBAR_SORT_MODES).default('alpha'),
   /** The pond default for the sidebar's page presentation (issue #108);
@@ -54,6 +69,7 @@ export const pondSettingsSchema = z.object({
   /** Per-pond opt-in to the built-in MCP endpoint (issue #105, default
    * off) — independent of the REST opt-in. */
   mcpEnabled: z.boolean().default(false),
+  theme: pondThemeSchema.default({}),
 });
 export type PondSettings = z.infer<typeof pondSettingsSchema>;
 
@@ -86,6 +102,7 @@ export const updatePondInputSchema = z
     commentPolicy: z.enum(COMMENT_POLICIES),
     apiEnabled: z.boolean(),
     mcpEnabled: z.boolean(),
+    theme: pondThemeSchema,
   })
   .partial();
 export type UpdatePondInput = z.infer<typeof updatePondInputSchema>;