diff --git a/web/src/i18n/en.json b/web/src/i18n/en.json
index f853041..ae7c008 100644
--- a/web/src/i18n/en.json
+++ b/web/src/i18n/en.json
@@ -9,6 +9,7 @@
   "form": { "selectPlaceholder": "— select —", "create": "Create object", "save": "Save", "cancel": "Cancel", "visibility": "Visibility", "draft": "Draft", "internal": "Internal", "required": "This field is required", "rejected": "The server rejected the changes — check required and referenced fields", "fieldRejected": "The field \"{{field}}\" was rejected — check its value", "flexibleHeading": "Catalogue fields" },
   "actions": { "edit": "Edit", "delete": "Delete", "rename": "Rename", "save": "Save", "closeDetail": "Close detail", "confirmDelete": "Delete this object? This cannot be undone.", "confirmDeleteTerm": "Delete this term? This cannot be undone.", "confirmDeleteAuthority": "Delete this authority? This cannot be undone.", "confirmDeleteField": "Delete this field definition? This cannot be undone.", "confirmDeleteVocabulary": "Delete this vocabulary? This cannot be undone.", "inUse": "Can't delete — used by {{count}} object(s). Clear those fields first." },
   "labels": { "label": "Label", "externalUri": "External URI (optional)", "otherLanguages": "This entry also has labels in other languages, which are kept." },
+  "theme": { "light": "Light", "dark": "Dark", "system": "System" },
   "vocab": {
     "newVocabulary": "New vocabulary", "key": "Key",
     "create": "Create", "selectPrompt": "Select a vocabulary to manage its terms",
diff --git a/web/src/i18n/sv.json b/web/src/i18n/sv.json
index e160c7e..fdfac42 100644
--- a/web/src/i18n/sv.json
+++ b/web/src/i18n/sv.json
@@ -9,6 +9,7 @@
   "form": { "selectPlaceholder": "— välj —", "create": "Skapa föremål", "save": "Spara", "cancel": "Avbryt", "visibility": "Synlighet", "draft": "Utkast", "internal": "Intern", "required": "Fältet är obligatoriskt", "rejected": "Servern avvisade ändringarna — kontrollera obligatoriska och refererade fält", "fieldRejected": "Fältet \"{{field}}\" avvisades — kontrollera värdet", "flexibleHeading": "Katalogfält" },
   "actions": { "edit": "Redigera", "delete": "Ta bort", "rename": "Byt namn", "save": "Spara", "closeDetail": "Stäng detalj", "confirmDelete": "Ta bort detta föremål? Detta kan inte ångras.", "confirmDeleteTerm": "Ta bort denna term? Detta kan inte ångras.", "confirmDeleteAuthority": "Ta bort denna auktoritet? Detta kan inte ångras.", "confirmDeleteField": "Ta bort denna fältdefinition? Detta kan inte ångras.", "confirmDeleteVocabulary": "Ta bort denna vokabulär? Detta kan inte ångras.", "inUse": "Kan inte ta bort — används av {{count}} föremål. Rensa de fälten först." },
   "labels": { "label": "Etikett", "externalUri": "Extern URI (valfritt)", "otherLanguages": "Denna post har även etiketter på andra språk, som behålls." },
+  "theme": { "light": "Ljust", "dark": "Mörkt", "system": "System" },
   "vocab": {
     "newVocabulary": "Ny vokabulär", "key": "Nyckel",
     "create": "Skapa", "selectPrompt": "Välj en vokabulär för att hantera dess termer",
diff --git a/web/src/shell/theme-switch.stories.tsx b/web/src/shell/theme-switch.stories.tsx
new file mode 100644
index 0000000..ae227c5
--- /dev/null
+++ b/web/src/shell/theme-switch.stories.tsx
@@ -0,0 +1,20 @@
+import type { Meta, StoryObj } from '@storybook/react-vite'
+import { expect } from 'storybook/test'
+
+import { ThemeSwitch } from './theme-switch'
+
+const meta = {
+  component: ThemeSwitch,
+  tags: ['ai-generated'],
+} satisfies Meta<typeof ThemeSwitch>
+
+export default meta
+type Story = StoryObj<typeof meta>
+
+export const Default: Story = {
+  play: async ({ canvas }) => {
+    await expect(canvas.getByRole('button', { name: /light/i })).toBeInTheDocument()
+    await expect(canvas.getByRole('button', { name: /dark/i })).toBeInTheDocument()
+    await expect(canvas.getByRole('button', { name: /system/i })).toBeInTheDocument()
+  },
+}
diff --git a/web/src/shell/theme-switch.test.tsx b/web/src/shell/theme-switch.test.tsx
new file mode 100644
index 0000000..deaa226
--- /dev/null
+++ b/web/src/shell/theme-switch.test.tsx
@@ -0,0 +1,57 @@
+import { afterEach, beforeEach, expect, test, vi } from "vitest";
+import { screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { renderApp } from "../test/render";
+import { ThemeSwitch } from "./theme-switch";
+
+beforeEach(() => {
+  vi.stubGlobal("matchMedia", (query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  }));
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  localStorage.clear();
+  document.documentElement.classList.remove("dark");
+});
+
+test("selecting Dark applies the dark class and persists", async () => {
+  renderApp(<ThemeSwitch />);
+  await userEvent.click(screen.getByRole("button", { name: /dark/i }));
+  expect(document.documentElement.classList.contains("dark")).toBe(true);
+  expect(localStorage.getItem("theme")).toBe("dark");
+  expect(screen.getByRole("button", { name: /dark/i })).toHaveAttribute("aria-pressed", "true");
+});
+
+test("selecting Light removes the dark class and persists", async () => {
+  localStorage.setItem("theme", "dark");
+  renderApp(<ThemeSwitch />);
+  await userEvent.click(screen.getByRole("button", { name: /light/i }));
+  expect(document.documentElement.classList.contains("dark")).toBe(false);
+  expect(localStorage.getItem("theme")).toBe("light");
+});
+
+test("selecting System resolves via prefers-color-scheme", async () => {
+  vi.stubGlobal("matchMedia", (query: string) => ({
+    matches: true,
+    media: query,
+    onchange: null,
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+    addListener: vi.fn(),
+    removeListener: vi.fn(),
+    dispatchEvent: vi.fn(),
+  }));
+  renderApp(<ThemeSwitch />);
+  await userEvent.click(screen.getByRole("button", { name: /system/i }));
+  expect(localStorage.getItem("theme")).toBe("system");
+  expect(document.documentElement.classList.contains("dark")).toBe(true);
+});
diff --git a/web/src/shell/theme-switch.tsx b/web/src/shell/theme-switch.tsx
new file mode 100644
index 0000000..e366376
--- /dev/null
+++ b/web/src/shell/theme-switch.tsx
@@ -0,0 +1,43 @@
+import { Monitor, Moon, Sun } from "lucide-react";
+import { useTranslation } from "react-i18next";
+
+import { useTheme } from "../theme/use-theme";
+import type { Theme } from "../theme/theme";
+import { cn } from "@/lib/utils";
+
+const OPTIONS: { value: Theme; Icon: typeof Sun }[] = [
+  { value: "light", Icon: Sun },
+  { value: "dark", Icon: Moon },
+  { value: "system", Icon: Monitor },
+];
+
+export function ThemeSwitch() {
+  const { t } = useTranslation();
+  const { theme, setTheme } = useTheme();
+
+  return (
+    <div className="flex gap-1">
+      {OPTIONS.map(({ value, Icon }) => {
+        const active = theme === value;
+        return (
+          <button
+            key={value}
+            type="button"
+            onClick={() => setTheme(value)}
+            aria-pressed={active}
+            aria-label={t(`theme.${value}`)}
+            title={t(`theme.${value}`)}
+            className={cn(
+              "rounded-md p-1 transition-colors",
+              active
+                ? "bg-accent text-foreground"
+                : "text-muted-foreground hover:text-foreground",
+            )}
+          >
+            <Icon className="h-4 w-4" aria-hidden />
+          </button>
+        );
+      })}
+    </div>
+  );
+}