diff --git a/src/components/SettingsView.tsx b/src/components/SettingsView.tsx
index ef9c62a..2c2632d 100644
--- a/src/components/SettingsView.tsx
+++ b/src/components/SettingsView.tsx
@@ -24,7 +24,7 @@ import {
 } from 'lucide-react';
 import { UserProfile } from '../types';
 import { fetchWithAuth } from '../lib/auth';
-import { applyTheme, getStoredTheme } from '../lib/theme';
+import { applyTheme, getStoredTheme, previewTheme, revertThemePreview, ThemeMode } from '../lib/theme';
 import { logoutUser } from '../lib/auth';
 
 interface SettingsViewProps {
@@ -52,7 +52,7 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
   const [email, setEmail] = useState(user?.email);
 
   // Appearance
-  const [appearance, setAppearance] = useState<'light' | 'dark' | 'system'>(() => user?.appearance || getStoredTheme());
+  const [appearance, setAppearance] = useState<ThemeMode>(() => user?.appearance || getStoredTheme());
 
   // Security & 2FA State
   const [currentPassword, setCurrentPassword] = useState('');
@@ -246,8 +246,8 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
     if (user) {
       setName(user.name || '');
       setEmail(user.email || '');
-      setAppearance(user.appearance || 'system');
-      applyTheme(user.appearance || 'system');
+      setAppearance(user.appearance || getStoredTheme());
+      applyTheme(user.appearance || getStoredTheme());
       setTwoFactorEnabled(user.twoFactorEnabled || false);
       setGeminiKey(user.apiKeys?.gemini || '');
       setOpenaiKey(user.apiKeys?.openai || '');
@@ -260,10 +260,16 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
       setAllowTelemetry(user.privacy?.allowTelemetry ?? true);
       setSearchHistoryCleared(user.privacy?.searchHistoryCleared ?? false);
     } else {
-      applyTheme(getStoredTheme());
+      revertThemePreview();
     }
   }, [user]);
 
+  useEffect(() => {
+    return () => {
+      revertThemePreview();
+    };
+  }, []);
+
   const handleExportData = async () => {
     setIsExporting(true);
     setExportSuccess(null);
@@ -356,9 +362,19 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
     }
   };
 
-  const handleAppearanceChange = (mode: 'light' | 'dark' | 'system') => {
+  const committedAppearance: ThemeMode = user?.appearance || getStoredTheme();
+
+  const handleSettingsTabChange = (id: typeof activeTab) => {
+    if (activeTab === 'appearance' && id !== 'appearance') {
+      setAppearance(committedAppearance);
+      previewTheme(committedAppearance);
+    }
+    setActiveTab(id);
+  };
+
+  const handleAppearanceChange = (mode: ThemeMode) => {
     setAppearance(mode);
-    applyTheme(mode);
+    previewTheme(mode);
   };
 
   const handleChangePassword = async (e: React.FormEvent) => {
@@ -543,6 +559,7 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
         }
       });
 
+      applyTheme(appearance);
       setSavedSuccess(true);
       setTimeout(() => setSavedSuccess(false), 3000);
     } catch (err: any) {
@@ -598,7 +615,7 @@ export default function SettingsView({ user, onUpdateUser }: SettingsViewProps)
             return (
               <button
                 key={item.id}
-                onClick={() => setActiveTab(item.id as any)}
+                onClick={() => handleSettingsTabChange(item.id as typeof activeTab)}
                 className={`w-full flex items-center gap-2.5 px-3 py-2 rounded-lg text-xs font-medium transition-all cursor-pointer text-left ${
                   isActive
                     ? 'bg-zinc-200 dark:bg-zinc-800 text-zinc-900 dark:text-white font-semibold'
diff --git a/src/lib/theme.test.ts b/src/lib/theme.test.ts
new file mode 100644
index 0000000..dab35e7
--- /dev/null
+++ b/src/lib/theme.test.ts
@@ -0,0 +1,143 @@
+import { describe, it, beforeEach } from 'node:test';
+import assert from 'node:assert/strict';
+
+function installThemeTestDom(options?: { prefersDark?: boolean }) {
+  const store: Record<string, string> = {};
+  const classes = new Set<string>();
+  const prefersDark = options?.prefersDark ?? false;
+  const changeListeners: Array<(event: { matches: boolean }) => void> = [];
+
+  const mediaQuery = {
+    matches: prefersDark,
+    addEventListener: (_event: string, handler: (event: { matches: boolean }) => void) => {
+      changeListeners.push(handler);
+    },
+    removeEventListener: () => {},
+    dispatch(matches: boolean) {
+      mediaQuery.matches = matches;
+      for (const handler of changeListeners) {
+        handler({ matches });
+      }
+    },
+  };
+
+  const localStorageMock = {
+    getItem: (key: string) => store[key] ?? null,
+    setItem: (key: string, value: string) => {
+      store[key] = value;
+    },
+    removeItem: (key: string) => {
+      delete store[key];
+    },
+    clear: () => {
+      Object.keys(store).forEach((key) => delete store[key]);
+    },
+  };
+
+  const classList = {
+    add: (cls: string) => {
+      classes.add(cls);
+    },
+    remove: (cls: string) => {
+      classes.delete(cls);
+    },
+    contains: (cls: string) => classes.has(cls),
+  };
+
+  (globalThis as any).localStorage = localStorageMock;
+  (globalThis as any).document = {
+    documentElement: { classList },
+  };
+  (globalThis as any).window = {
+    matchMedia: (query: string) =>
+      query.includes('prefers-color-scheme: dark')
+        ? mediaQuery
+        : { matches: false, addEventListener: () => {}, removeEventListener: () => {} },
+  };
+
+  return { store, classes, mediaQuery };
+}
+
+describe('theme preview vs persist', () => {
+  beforeEach(() => {
+    installThemeTestDom();
+  });
+
+  it('previewTheme applies dark class without writing localStorage', async () => {
+    const { store, classes } = installThemeTestDom();
+    const { previewTheme, getStoredTheme } = await import('./theme.ts');
+
+    previewTheme('dark');
+
+    assert.equal(classes.has('dark'), true);
+    assert.equal(store.theme, undefined);
+    assert.equal(getStoredTheme(), 'light');
+  });
+
+  it('applyTheme persists the mode and applies it', async () => {
+    const { store, classes } = installThemeTestDom();
+    const { applyTheme, getStoredTheme } = await import('./theme.ts');
+
+    applyTheme('dark');
+
+    assert.equal(store.theme, 'dark');
+    assert.equal(getStoredTheme(), 'dark');
+    assert.equal(classes.has('dark'), true);
+  });
+
+  it('revertThemePreview restores the last saved theme after an unsaved preview', async () => {
+    const { store, classes } = installThemeTestDom();
+    const { applyTheme, previewTheme, revertThemePreview, getStoredTheme } = await import('./theme.ts');
+
+    applyTheme('light');
+    previewTheme('dark');
+    assert.equal(classes.has('dark'), true);
+    assert.equal(getStoredTheme(), 'light');
+
+    revertThemePreview();
+
+    assert.equal(store.theme, 'light');
+    assert.equal(classes.has('dark'), false);
+    assert.equal(getStoredTheme(), 'light');
+  });
+
+  it('previewing system theme does not commit until applyTheme', async () => {
+    const { store, classes, mediaQuery } = installThemeTestDom({ prefersDark: true });
+    const { applyTheme, previewTheme, initTheme, getStoredTheme } = await import('./theme.ts');
+
+    initTheme();
+    applyTheme('light');
+    previewTheme('system');
+
+    assert.equal(classes.has('dark'), true);
+    assert.equal(store.theme, 'light');
+    assert.equal(getStoredTheme(), 'light');
+
+    mediaQuery.dispatch(false);
+    assert.equal(classes.has('dark'), false, 'unsaved system preview should follow OS light');
+    assert.equal(store.theme, 'light');
+
+    mediaQuery.dispatch(true);
+    assert.equal(classes.has('dark'), true, 'unsaved system preview should follow OS dark');
+    assert.equal(store.theme, 'light');
+  });
+
+  it('OS color-scheme changes ignore the listener when the active preview is not system', async () => {
+    const { store, classes, mediaQuery } = installThemeTestDom({ prefersDark: false });
+    const { applyTheme, previewTheme, initTheme, revertThemePreview } = await import('./theme.ts');
+
+    initTheme();
+    applyTheme('system');
+    previewTheme('dark');
+
+    mediaQuery.dispatch(false);
+    assert.equal(classes.has('dark'), true, 'unsaved dark preview should ignore OS going light');
+    assert.equal(store.theme, 'system');
+
+    revertThemePreview();
+    assert.equal(classes.has('dark'), false);
+
+    mediaQuery.dispatch(true);
+    assert.equal(classes.has('dark'), true, 'reverted system theme should follow OS again');
+  });
+});
diff --git a/src/lib/theme.ts b/src/lib/theme.ts
index d53f4de..755c705 100644
--- a/src/lib/theme.ts
+++ b/src/lib/theme.ts
@@ -1,5 +1,8 @@
 export type ThemeMode = 'light' | 'dark' | 'system';
 
+/** Currently applied theme, including unsaved previews. */
+let activeThemeMode: ThemeMode = 'light';
+
 export function getStoredTheme(): ThemeMode {
   const stored = localStorage.getItem('theme');
   if (stored === 'light' || stored === 'dark' || stored === 'system') {
@@ -8,31 +11,42 @@ export function getStoredTheme(): ThemeMode {
   return 'light'; // Default to light mode as requested
 }
 
-export function applyTheme(mode: ThemeMode) {
-  localStorage.setItem('theme', mode);
-  const isDark =
+function isDarkMode(mode: ThemeMode): boolean {
+  return (
     mode === 'dark' ||
-    (mode === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+    (mode === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches)
+  );
+}
 
-  if (isDark) {
+/** Apply a theme to the document without persisting it. */
+export function previewTheme(mode: ThemeMode) {
+  activeThemeMode = mode;
+  if (isDarkMode(mode)) {
     document.documentElement.classList.add('dark');
   } else {
     document.documentElement.classList.remove('dark');
   }
 }
 
+/** Persist the theme and apply it. */
+export function applyTheme(mode: ThemeMode) {
+  localStorage.setItem('theme', mode);
+  previewTheme(mode);
+}
+
+/** Restore the last saved theme (localStorage / default). */
+export function revertThemePreview() {
+  previewTheme(getStoredTheme());
+}
+
 export function initTheme() {
   const current = getStoredTheme();
   applyTheme(current);
 
-  // Listen for system theme changes if set to 'system'
-  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', (e) => {
-    if (getStoredTheme() === 'system') {
-      if (e.matches) {
-        document.documentElement.classList.add('dark');
-      } else {
-        document.documentElement.classList.remove('dark');
-      }
+  // Follow OS changes only while the active (possibly previewed) mode is system.
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', () => {
+    if (activeThemeMode === 'system') {
+      previewTheme('system');
     }
   });
 }