diff --git a/index.html b/index.html
index a5734ba..5e7a099 100644
--- a/index.html
+++ b/index.html
@@ -46,6 +46,21 @@
 		<!-- Preconnect -->
 		<link rel="preconnect" href="https://fonts.googleapis.com" />
 		<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
+
+		<!--
+			#750: Applies the stored theme class to <html> before the first
+			paint so there is no flash of the wrong theme on load.
+			Reads localStorage['theme']; falls back to prefers-color-scheme.
+		-->
+		<script>
+			(function () {
+				var stored = localStorage.getItem('theme');
+				var prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+				if (stored === 'dark' || (!stored && prefersDark)) {
+					document.documentElement.classList.add('dark');
+				}
+			})();
+		</script>
 	</head>
 	<body>
 		<!--
diff --git a/src/components/home/Header.tsx b/src/components/home/Header.tsx
index f88f65e..aec0d6d 100644
--- a/src/components/home/Header.tsx
+++ b/src/components/home/Header.tsx
@@ -1,7 +1,9 @@
 import { useEffect, useState } from 'react';
+import { Moon, Sun } from 'lucide-react';
 import WalletStatusChip from '@/components/common/WalletStatusChip';
 import NotificationBell from '@/components/common/NotificationBell';
 import { useProfileStore } from '@/hooks/useProfileStore';
+import { useTheme } from '@/hooks/useTheme';
 import { Link } from 'react-router';
 
 const navLinks = [
@@ -13,6 +15,7 @@ const navLinks = [
 export default function Header() {
 	const [scrolled, setScrolled] = useState(false);
 	const profile = useProfileStore(state => state.profile);
+	const { theme, toggleTheme } = useTheme();
 
 	useEffect(() => {
 		const onScroll = () => {
@@ -69,9 +72,25 @@ export default function Header() {
 					)}
 				</nav>
 
-				{/* Right-side actions: notification bell (#720) + wallet status chip (#686).
+				{/* Right-side actions: dark mode toggle (#750) + notification bell (#720) + wallet status chip (#686).
 				    NotificationBell is only rendered when a user profile is available. */}
 				<div className="flex items-center gap-2">
+					<button
+						type="button"
+						onClick={toggleTheme}
+						aria-label={theme === 'dark' ? 'Switch to light mode' : 'Switch to dark mode'}
+						className={`rounded-md p-1.5 transition-colors duration-200 ${
+							scrolled
+								? 'text-gray-600 hover:bg-black/5 hover:text-gray-900'
+								: 'text-white/60 hover:text-white/90'
+						}`}
+					>
+						{theme === 'dark' ? (
+							<Sun className="size-4" aria-hidden="true" />
+						) : (
+							<Moon className="size-4" aria-hidden="true" />
+						)}
+					</button>
 					{profile && (
 						<NotificationBell
 							userId={profile.id}
diff --git a/src/hooks/__tests__/useTheme.test.ts b/src/hooks/__tests__/useTheme.test.ts
new file mode 100644
index 0000000..a1cf515
--- /dev/null
+++ b/src/hooks/__tests__/useTheme.test.ts
@@ -0,0 +1,163 @@
+import { act, renderHook } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { THEME_STORAGE_KEY, useTheme } from '@/hooks/useTheme';
+
+const localStorageStub = (() => {
+	let store: Record<string, string> = {};
+	return {
+		getItem: (k: string) => store[k] ?? null,
+		setItem: (k: string, v: string) => { store[k] = v; },
+		removeItem: (k: string) => { delete store[k]; },
+		clear: () => { store = {}; },
+	};
+})();
+
+function setOsPreference(prefersDark: boolean) {
+	Object.defineProperty(window, 'matchMedia', {
+		writable: true,
+		value: vi.fn().mockReturnValue({
+			matches: prefersDark,
+			addEventListener: vi.fn(),
+			removeEventListener: vi.fn(),
+		}),
+	});
+}
+
+beforeEach(() => {
+	localStorageStub.clear();
+	vi.stubGlobal('localStorage', localStorageStub);
+	document.documentElement.classList.remove('dark');
+	setOsPreference(false);
+});
+
+afterEach(() => {
+	vi.unstubAllGlobals();
+});
+
+describe('useTheme initial value', () => {
+	it('defaults to light when localStorage is empty and OS prefers light', () => {
+		setOsPreference(false);
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('light');
+	});
+
+	it('defaults to dark when localStorage is empty and OS prefers dark', () => {
+		setOsPreference(true);
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('dark');
+	});
+
+	it('reads stored light theme from localStorage over OS dark preference', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'light');
+		setOsPreference(true);
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('light');
+	});
+
+	it('reads stored dark theme from localStorage over OS light preference', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'dark');
+		setOsPreference(false);
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('dark');
+	});
+
+	it('ignores an invalid localStorage value and falls back to OS preference', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'system');
+		setOsPreference(true);
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('dark');
+	});
+});
+
+describe('useTheme html class management', () => {
+	it('adds the dark class to <html> when theme is dark', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'dark');
+		renderHook(() => useTheme());
+		expect(document.documentElement.classList.contains('dark')).toBe(true);
+	});
+
+	it('removes the dark class from <html> when theme is light', () => {
+		document.documentElement.classList.add('dark');
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'light');
+		renderHook(() => useTheme());
+		expect(document.documentElement.classList.contains('dark')).toBe(false);
+	});
+
+	it('adds the dark class after toggling from light to dark', () => {
+		const { result } = renderHook(() => useTheme());
+		expect(document.documentElement.classList.contains('dark')).toBe(false);
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(document.documentElement.classList.contains('dark')).toBe(true);
+	});
+
+	it('removes the dark class after toggling from dark to light', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'dark');
+		const { result } = renderHook(() => useTheme());
+		expect(document.documentElement.classList.contains('dark')).toBe(true);
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(document.documentElement.classList.contains('dark')).toBe(false);
+	});
+});
+
+describe('useTheme localStorage persistence', () => {
+	it('writes the initial theme to localStorage on mount', () => {
+		setOsPreference(false);
+		renderHook(() => useTheme());
+		expect(localStorageStub.getItem(THEME_STORAGE_KEY)).toBe('light');
+	});
+
+	it('persists dark theme to localStorage after toggling', () => {
+		const { result } = renderHook(() => useTheme());
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(localStorageStub.getItem(THEME_STORAGE_KEY)).toBe('dark');
+	});
+
+	it('persists light theme to localStorage after toggling back', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'dark');
+		const { result } = renderHook(() => useTheme());
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(localStorageStub.getItem(THEME_STORAGE_KEY)).toBe('light');
+	});
+});
+
+describe('useTheme toggleTheme', () => {
+	it('switches from light to dark', () => {
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('light');
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(result.current.theme).toBe('dark');
+	});
+
+	it('switches from dark to light', () => {
+		localStorageStub.setItem(THEME_STORAGE_KEY, 'dark');
+		const { result } = renderHook(() => useTheme());
+		expect(result.current.theme).toBe('dark');
+
+		act(() => { result.current.toggleTheme(); });
+
+		expect(result.current.theme).toBe('light');
+	});
+
+	it('toggles back and forth correctly', () => {
+		const { result } = renderHook(() => useTheme());
+
+		act(() => { result.current.toggleTheme(); });
+		expect(result.current.theme).toBe('dark');
+
+		act(() => { result.current.toggleTheme(); });
+		expect(result.current.theme).toBe('light');
+
+		act(() => { result.current.toggleTheme(); });
+		expect(result.current.theme).toBe('dark');
+	});
+});
diff --git a/src/hooks/useTheme.ts b/src/hooks/useTheme.ts
new file mode 100644
index 0000000..76d40d6
--- /dev/null
+++ b/src/hooks/useTheme.ts
@@ -0,0 +1,35 @@
+import { useCallback, useEffect, useState } from 'react';
+
+export type Theme = 'light' | 'dark';
+
+export const THEME_STORAGE_KEY = 'theme';
+
+function resolveInitialTheme(): Theme {
+	const stored = localStorage.getItem(THEME_STORAGE_KEY);
+	if (stored === 'light' || stored === 'dark') return stored;
+	return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}
+
+function applyTheme(theme: Theme): void {
+	document.documentElement.classList.toggle('dark', theme === 'dark');
+}
+
+export interface UseThemeResult {
+	theme: Theme;
+	toggleTheme: () => void;
+}
+
+export function useTheme(): UseThemeResult {
+	const [theme, setTheme] = useState<Theme>(resolveInitialTheme);
+
+	useEffect(() => {
+		applyTheme(theme);
+		localStorage.setItem(THEME_STORAGE_KEY, theme);
+	}, [theme]);
+
+	const toggleTheme = useCallback(() => {
+		setTheme(prev => (prev === 'light' ? 'dark' : 'light'));
+	}, []);
+
+	return { theme, toggleTheme };
+}
