diff --git a/CLAUDE.md b/CLAUDE.md
index 045235be..1bca6f0f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -29,6 +29,7 @@ SvelteKit 2 + Svelte 5 (runes) SPA using `@sveltejs/adapter-static`. **`docs/` i
 
 - `inputState` — a `$state` rune holding `State` (code, mermaid config JSON, pan/zoom, etc.), initialized from localStorage key `codeStore`. **All writes must go through the exported update functions** (`updateCode`, `updateCodeStore`, `updateConfig`, `replaceInputState`, ...), which call `persistAndProcess()`: persist to localStorage → async re-validate via `mermaid.parse` → publish to `validatedState`. Reads inside update functions are wrapped in `untrack` so calling effects don't depend on the whole input state.
 - `validatedState` — read-only validated snapshot (adds `error`, `errorMarkers`, `serialized`, `diagramType`). Internal reads should use this, but it is never persisted or shared externally.
+- The diagram `theme` in the config is editor-managed: after every validation and on site light/dark changes, `syncManagedTheme` sets it to the diagram type's mermaid default (its config section's `theme`, e.g. `redux-color` for flowcharts) or the redux `-dark` variant / `dark` in dark mode. A theme outside that family (`isManagedTheme` in `mermaid.ts`) is the user's choice and is never touched. The default config is therefore `{}` until the first validation fills the theme in.
 - The serialized state is mirrored into the URL hash (debounced) — the URL **is** the sharing mechanism. `src/lib/util/serde.ts` encodes state as `pako:<base64 deflate>` (legacy plain-base64 supported). `mermaid.ink` PNG/SVG links, Kroki, and Mermaid Chart URLs are derived from it in `urls`.
 - App startup goes through `initHandler()` in `src/lib/util/util.ts`: migrations → load state from URL hash → optional gist/URL loading (`fileLoaders/`) → start URL-hash subscription → analytics.
 
diff --git a/src/lib/util/mermaid.test.ts b/src/lib/util/mermaid.test.ts
index 7e8e4c73..98d1b8ee 100644
--- a/src/lib/util/mermaid.test.ts
+++ b/src/lib/util/mermaid.test.ts
@@ -1,5 +1,47 @@
 import { describe, expect, it } from 'vitest';
-import { getSampleDiagrams } from './mermaid';
+import { darkVariantOf, getDefaultTheme, getSampleDiagrams, isManagedTheme } from './mermaid';
+
+describe('getDefaultTheme', () => {
+  it.each([
+    ['flowchart-v2', 'redux-color'],
+    ['flowchart-elk', 'redux-color'],
+    ['classDiagram', 'redux-color'],
+    ['stateDiagram', 'redux-color'],
+    ['sequence', 'redux-color'],
+    ['pie', 'default'],
+    ['xychart', 'default'],
+    ['railroadAbnf', 'default'],
+    ['wardley', 'default']
+  ])('resolves %s to its config section default (%s)', (diagramType, theme) => {
+    expect(getDefaultTheme(diagramType)).toBe(theme);
+  });
+});
+
+describe('darkVariantOf', () => {
+  it.each([
+    ['redux-color', 'redux-dark-color'],
+    ['redux', 'redux-dark'],
+    ['redux-dark-color', 'redux-dark-color'],
+    ['default', 'dark'],
+    ['forest', 'dark']
+  ])('%s → %s', (theme, dark) => {
+    expect(darkVariantOf(theme)).toBe(dark);
+  });
+});
+
+describe('isManagedTheme', () => {
+  it('treats a missing theme and every derivable default as editor-managed', () => {
+    for (const theme of [undefined, 'default', 'dark', 'redux-color', 'redux-dark-color']) {
+      expect(isManagedTheme(theme), String(theme)).toBe(true);
+    }
+  });
+
+  it('treats any other theme as the user’s choice', () => {
+    for (const theme of ['forest', 'neutral', 'neo', 'base', 'redux', 42]) {
+      expect(isManagedTheme(theme), String(theme)).toBe(false);
+    }
+  });
+});
 
 describe('getSampleDiagrams', () => {
   const samples = getSampleDiagrams();
diff --git a/src/lib/util/mermaid.ts b/src/lib/util/mermaid.ts
index 3e626035..e9f58e33 100644
--- a/src/lib/util/mermaid.ts
+++ b/src/lib/util/mermaid.ts
@@ -29,6 +29,64 @@ export const parse = async (code: string) => {
  */
 export const defaultMermaidConfig = mermaid.mermaidAPI.defaultConfig ?? {};
 
+// Detector ids (what `mermaid.parse` reports as diagramType) whose config
+// section is named differently. Every other id shares its section's name, and
+// ids without a section at all (wardley, zenuml) use the global defaults.
+const CONFIG_SECTION_ALIASES: Record<string, string> = {
+  classDiagram: 'class',
+  'flowchart-elk': 'flowchart',
+  'flowchart-v2': 'flowchart',
+  railroadAbnf: 'railroad',
+  railroadEbnf: 'railroad',
+  railroadPeg: 'railroad',
+  stateDiagram: 'state',
+  xychart: 'xyChart'
+};
+
+const themeOfSection = (section: unknown): string | undefined => {
+  if (section && typeof section === 'object' && 'theme' in section) {
+    const { theme } = section as { theme?: unknown };
+    if (typeof theme === 'string') {
+      return theme;
+    }
+  }
+  return undefined;
+};
+
+const globalDefaultTheme: string = defaultMermaidConfig.theme ?? 'default';
+
+/** Mermaid's own default theme for a diagram type: its config section's, else the global one. */
+export const getDefaultTheme = (diagramType: string): string => {
+  const key = CONFIG_SECTION_ALIASES[diagramType] ?? diagramType;
+  const section = (defaultMermaidConfig as Record<string, unknown>)[key];
+  return themeOfSection(section) ?? globalDefaultTheme;
+};
+
+/** Dark counterpart of a default theme: redux themes have -dark variants, anything else uses `dark`. */
+export const darkVariantOf = (theme: string): string => {
+  if (theme.includes('dark')) {
+    return theme;
+  }
+  return theme.startsWith('redux') ? theme.replace('redux', 'redux-dark') : 'dark';
+};
+
+// Every theme the editor may set on its own: the global default, each config
+// section's default and their dark variants. Anything else is the user's choice.
+const managedThemes = new Set<string>();
+for (const theme of [
+  globalDefaultTheme,
+  ...Object.values(defaultMermaidConfig).map(themeOfSection)
+]) {
+  if (theme) {
+    managedThemes.add(theme);
+    managedThemes.add(darkVariantOf(theme));
+  }
+}
+
+/** Whether the editor may replace this theme (a missing theme counts as managed). */
+export const isManagedTheme = (theme: unknown): boolean =>
+  theme === undefined || (typeof theme === 'string' && managedThemes.has(theme));
+
 export const standardizeDiagramType = (diagramType: string) => {
   switch (diagramType) {
     case 'class':
diff --git a/src/lib/util/state.svelte.test.ts b/src/lib/util/state.svelte.test.ts
index 16bab07e..1b25e142 100644
--- a/src/lib/util/state.svelte.test.ts
+++ b/src/lib/util/state.svelte.test.ts
@@ -1,6 +1,6 @@
 import type { State } from '$lib/types';
 import { flushSync } from 'svelte';
-import { describe, expect, it } from 'vitest';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
 import {
   clearDefaultThemeConfig,
   defaultState,
@@ -11,6 +11,7 @@ import {
   updateCode,
   updateCodeStore,
   updateConfig,
+  validatedState,
   verifyState
 } from './state.svelte';
 
@@ -94,44 +95,107 @@ describe('default config', () => {
   });
 });
 
-describe('toggleDarkTheme', () => {
-  it('sets the dark theme when the config has no theme', () => {
+// Validation (mermaid.parse) is asynchronous; these helpers wait for it.
+const themeOf = () => (JSON.parse(inputState.mermaid) as { theme?: string }).theme;
+const waitForTheme = (theme: string | undefined) =>
+  vi.waitFor(() => expect(themeOf()).toBe(theme), { timeout: 10_000 });
+const settled = () =>
+  vi.waitFor(
+    () => {
+      expect(validatedState.current.code).toBe(inputState.code);
+      expect(validatedState.current.mermaid).toBe(inputState.mermaid);
+    },
+    { timeout: 10_000 }
+  );
+
+const flowchart = 'graph TD\n  A[Managed] --> B[Theme]';
+const pie = 'pie\n  "A": 1\n  "B": 2';
+const slow = { timeout: 15_000 };
+
+describe('managed theme', () => {
+  it('derives the diagram type default once the code validates', slow, async () => {
+    toggleDarkTheme(false);
     updateConfig('{}');
+    updateCode(flowchart);
+    await waitForTheme('redux-color');
+    expect(readStoredState().mermaid).toBe('{\n  "theme": "redux-color"\n}');
+  });
+
+  it('follows the diagram type when the code changes', slow, async () => {
+    toggleDarkTheme(false);
+    updateConfig('{}');
+    updateCode(flowchart);
+    await waitForTheme('redux-color');
+    updateCode(pie);
+    await waitForTheme('default');
+  });
+
+  it('uses the redux dark variant in dark mode and switches back', slow, async () => {
+    updateConfig('{}');
+    updateCode(flowchart);
     toggleDarkTheme(true);
-    expect(JSON.parse(inputState.mermaid)).toEqual({ theme: 'dark' });
-  });
-
-  it('removes the theme again in light mode instead of pinning "default"', () => {
-    updateConfig('{\n  "theme": "dark"\n}');
+    await waitForTheme('redux-dark-color');
     toggleDarkTheme(false);
-    expect(inputState.mermaid).toBe('{}');
-    expect(readStoredState().mermaid).toBe('{}');
+    await waitForTheme('redux-color');
   });
 
-  it('drops a pinned "default" theme in light mode', () => {
+  it('falls back to the dark theme for diagrams without a redux default', slow, async () => {
+    updateConfig('{}');
+    updateCode(pie);
+    toggleDarkTheme(true);
+    await waitForTheme('dark');
+    toggleDarkTheme(false);
+    await waitForTheme('default');
+  });
+
+  it('replaces the legacy pinned "default" theme', slow, async () => {
+    toggleDarkTheme(false);
+    updateCode(flowchart);
     updateConfig('{\n  "theme": "default"\n}');
-    toggleDarkTheme(false);
-    expect(inputState.mermaid).toBe('{}');
+    await waitForTheme('redux-color');
   });
 
-  it('leaves a user-chosen theme alone in both modes', () => {
+  it('leaves a user-chosen theme alone', slow, async () => {
+    updateCode(flowchart);
     updateConfig('{\n  "theme": "forest"\n}');
     toggleDarkTheme(true);
-    expect(JSON.parse(inputState.mermaid)).toEqual({ theme: 'forest' });
+    await settled();
+    expect(themeOf()).toBe('forest');
+    updateCode(pie);
+    await settled();
+    expect(themeOf()).toBe('forest');
     toggleDarkTheme(false);
-    expect(JSON.parse(inputState.mermaid)).toEqual({ theme: 'forest' });
+    await settled();
+    expect(themeOf()).toBe('forest');
   });
 
-  it('keeps the other config keys when changing the theme', () => {
-    updateConfig('{\n  "theme": "default",\n  "look": "neo"\n}');
-    toggleDarkTheme(true);
-    expect(JSON.parse(inputState.mermaid)).toEqual({ look: 'neo', theme: 'dark' });
+  it('keeps the other config keys', slow, async () => {
+    toggleDarkTheme(false);
+    updateCode(flowchart);
+    updateConfig('{\n  "look": "classic"\n}');
+    await waitForTheme('redux-color');
+    expect(JSON.parse(inputState.mermaid)).toEqual({ look: 'classic', theme: 'redux-color' });
+  });
+
+  it('does nothing while the config is not valid JSON', slow, async () => {
+    updateCode(flowchart);
+    updateConfig('{ "theme": ');
+    expect(() => toggleDarkTheme(true)).not.toThrow();
+    await settled();
+    expect(inputState.mermaid).toBe('{ "theme": ');
     toggleDarkTheme(false);
-    expect(JSON.parse(inputState.mermaid)).toEqual({ look: 'neo' });
   });
 });
 
 describe('clearDefaultThemeConfig migration', () => {
+  // Code that does not parse has no diagram type, which keeps the managed
+  // theme sync out of the way so the migration is observed on its own.
+  beforeEach(async () => {
+    toggleDarkTheme(false);
+    updateCode('not a diagram');
+    await settled();
+  });
+
   it('clears a config that only pins the "default" theme', () => {
     updateConfig('{\n  "theme": "default"\n}');
     clearDefaultThemeConfig();
diff --git a/src/lib/util/state.svelte.ts b/src/lib/util/state.svelte.ts
index 72324225..946ccc71 100644
--- a/src/lib/util/state.svelte.ts
+++ b/src/lib/util/state.svelte.ts
@@ -10,7 +10,7 @@ import {
   findMostRelevantLineNumber,
   replaceLineNumberInErrorMessage
 } from './errorHandling';
-import { parse } from './mermaid';
+import { darkVariantOf, getDefaultTheme, isManagedTheme, parse } from './mermaid';
 import { readJSON, writeJSON } from './persist.svelte';
 import { findUnsafeConfigPaths, stripConfigPaths } from './sanitize';
 import { deserializeState, pakoSerde, serializeState } from './serde';
@@ -125,6 +125,7 @@ const persistAndProcess = (): void => {
   void processState(snapshot).then((processed) => {
     validatedCurrent = processed;
     updateHash?.(processed.serialized);
+    syncManagedTheme(processed.diagramType);
   });
 };
 
@@ -277,25 +278,46 @@ export const updateConfig = (config: string): void => {
   updateCodeStore({ mermaid: config });
 };
 
-// The site's light/dark mode only manages a theme it set itself: none, `dark`,
-// or the `default` older versions seeded. Anything else is the user's choice.
-// Light mode removes the theme instead of pinning `default`, so mermaid's own
-// per-diagram defaults apply.
-export const toggleDarkTheme = (dark: boolean): void => {
-  update((state) => {
-    const config = JSON.parse(state.mermaid) as MermaidConfig;
-    if (config.theme && !['dark', 'default'].includes(config.theme)) {
+let siteDark = false;
+
+// The editor manages the diagram theme unless the user picked one of their
+// own. A missing theme, mermaid's global default, a config section's default
+// or any of their dark variants (see isManagedTheme) is replaced by the
+// current diagram type's default, or its dark counterpart while the site is
+// dark. Runs after every validation, since the diagram type may have changed,
+// and whenever the site mode changes. Converges after one update: the next
+// validation finds the theme already in place. Reads are untracked so an
+// effect calling toggleDarkTheme does not subscribe to the input state.
+const syncManagedTheme = (diagramType: string | undefined): void => {
+  if (!diagramType) {
+    return;
+  }
+  untrack(() => {
+    let config: MermaidConfig;
+    try {
+      config = JSON.parse(input.mermaid) as MermaidConfig;
+    } catch {
       return;
     }
-    if (dark) {
-      config.theme = 'dark';
-    } else {
-      delete config.theme;
+    if (!isManagedTheme(config.theme)) {
+      return;
     }
-    state.mermaid = formatJSON(config);
+    const defaultTheme = getDefaultTheme(diagramType);
+    const theme = siteDark ? darkVariantOf(defaultTheme) : defaultTheme;
+    if (config.theme === theme) {
+      return;
+    }
+    update((state) => {
+      state.mermaid = formatJSON({ ...config, theme });
+    });
   });
 };
 
+export const toggleDarkTheme = (dark: boolean): void => {
+  siteDark = dark;
+  syncManagedTheme(untrack(() => validatedCurrent.diagramType));
+};
+
 const isOnlyDefaultTheme = (config: string): boolean => {
   try {
     const parsed: unknown = JSON.parse(config);
diff --git a/tests/configMigration.spec.ts b/tests/configMigration.spec.ts
index 866d05af..7442f35b 100644
--- a/tests/configMigration.spec.ts
+++ b/tests/configMigration.spec.ts
@@ -43,16 +43,17 @@ const migrationVersion = (page: Page) =>
   );
 
 test.describe('Default theme config migration', () => {
-  test('clears a legacy config that only pins the default theme', async ({ page }) => {
+  test('replaces a legacy default-theme config with the diagram default', async ({ page }) => {
     await page.emulateMedia({ colorScheme: 'light' });
     await seedLegacyInstall(page);
     await openEditor(page);
     await expect(page.locator('#view')).toContainText('Legacy');
     await expect.poll(() => migrationVersion(page)).toBe(1);
-    await expect.poll(() => storedConfig(page)).toBe('{}');
+    // A flowchart: mermaid 12's own default for it is redux-color.
+    await expect.poll(() => storedConfig(page)).toBe('{\n  "theme": "redux-color"\n}');
   });
 
-  test('switches the cleared config to the dark theme when the site is dark', async ({ page }) => {
+  test('uses the redux dark variant when the site is dark', async ({ page }) => {
     await page.emulateMedia({ colorScheme: 'dark' });
     await seedLegacyInstall(page);
     await openEditor(page);
@@ -60,6 +61,6 @@ test.describe('Default theme config migration', () => {
     await expect.poll(() => migrationVersion(page)).toBe(1);
     await expect
       .poll(async () => JSON.parse((await storedConfig(page)) ?? '{}') as unknown)
-      .toEqual({ theme: 'dark' });
+      .toEqual({ theme: 'redux-dark-color' });
   });
 });
diff --git a/tests/loadSite.spec.ts b/tests/loadSite.spec.ts
index f94b03f5..dce4543d 100644
--- a/tests/loadSite.spec.ts
+++ b/tests/loadSite.spec.ts
@@ -100,6 +100,8 @@ test.describe('Site Loads', () => {
     const parsedConfig = JSON.parse(parsedStore.mermaid) as Record<string, unknown>;
     expect(parsedConfig).toEqual({
       someOtherSetting: 'Test value',
+      // The editor fills in the diagram type's default theme (a flowchart here).
+      theme: 'redux-color',
       themeVariables: {}
     });
     // should scrub unsafe securityLevel but keep other settings
diff --git a/tests/managedTheme.spec.ts b/tests/managedTheme.spec.ts
new file mode 100644
index 00000000..6222cad2
--- /dev/null
+++ b/tests/managedTheme.spec.ts
@@ -0,0 +1,32 @@
+import type { Page } from '@playwright/test';
+import { expect, test } from './test';
+
+const storedTheme = (page: Page) =>
+  page.evaluate(() => {
+    const { mermaid } = JSON.parse(localStorage.getItem('codeStore') ?? '{}') as {
+      mermaid?: string;
+    };
+    return (JSON.parse(mermaid ?? '{}') as { theme?: string }).theme;
+  });
+
+test.describe('Managed theme', () => {
+  test.beforeEach(async ({ editPage }) => {
+    await editPage.toggleSampleDiagrams();
+  });
+
+  test('follows the diagram type', async ({ editPage, page }) => {
+    await editPage.loadSampleDiagram('Pie');
+    await expect.poll(() => storedTheme(page)).toBe('default');
+    await editPage.loadSampleDiagram('Flowchart');
+    await expect.poll(() => storedTheme(page)).toBe('redux-color');
+  });
+
+  test('switches to the dark variant with the site theme', async ({ editPage, page }) => {
+    await editPage.loadSampleDiagram('Flowchart');
+    await expect.poll(() => storedTheme(page)).toBe('redux-color');
+    await editPage.toggleTheme();
+    await expect.poll(() => storedTheme(page)).toBe('redux-dark-color');
+    await editPage.toggleTheme();
+    await expect.poll(() => storedTheme(page)).toBe('redux-color');
+  });
+});