diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index fe28da81..b8499280 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -5,6 +5,7 @@ import i18n from './i18n'
 import { WalletProvider } from './context/WalletContext'
 import { ToastProvider } from './context/ToastContext'
 import { NotificationProvider } from './context/NotificationContext'
+import { ThemeProvider } from './context/ThemeContext'
 import { NotFoundPage } from './pages/NotFoundPage'
 import AppShell from './components/layout/AppShell'
 import LandingPage from './pages/LandingPage'
@@ -92,13 +93,15 @@ const App: React.FC = () => {
   return (
     <I18nextProvider i18n={i18n}>
       <ErrorBoundary>
-        <WalletProvider>
-          <ToastProvider>
-            <Router>
-              <AppContent />
-            </Router>
-          </ToastProvider>
-        </WalletProvider>
+        <ThemeProvider>
+          <WalletProvider>
+            <ToastProvider>
+              <Router>
+                <AppContent />
+              </Router>
+            </ToastProvider>
+          </WalletProvider>
+        </ThemeProvider>
       </ErrorBoundary>
     </I18nextProvider>
   )
diff --git a/frontend/src/components/layout/TopNav.test.tsx b/frontend/src/components/layout/TopNav.test.tsx
new file mode 100644
index 00000000..492610a9
--- /dev/null
+++ b/frontend/src/components/layout/TopNav.test.tsx
@@ -0,0 +1,49 @@
+import { render, screen, fireEvent, act } from '@testing-library/react'
+import { MemoryRouter } from 'react-router-dom'
+import { describe, test, expect, beforeEach, vi } from 'vitest'
+import TopNav from './TopNav'
+import { WalletProvider } from '../../context/WalletContext'
+import { ThemeProvider } from '../../context/ThemeContext'
+
+describe('TopNav Theme Toggle', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    // minimal matchMedia mock
+    Object.defineProperty(window, 'matchMedia', {
+      writable: true,
+      value: vi.fn().mockImplementation((query) => ({
+        matches: false,
+        media: query,
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+        addListener: vi.fn(),
+        removeListener: vi.fn(),
+      })),
+    })
+  })
+
+  test('cycles theme and persists selection to localStorage', async () => {
+    render(
+      <MemoryRouter>
+        <WalletProvider>
+          <ThemeProvider>
+            <TopNav onMenuClick={vi.fn()} onToggleSidebar={vi.fn()} sidebarCollapsed={false} isMobile={false} />
+          </ThemeProvider>
+        </WalletProvider>
+      </MemoryRouter>
+    )
+
+    const toggle = screen.getByRole('switch')
+    // default is dark
+    expect(localStorage.getItem('theme-mode')).toBe('dark')
+
+    await act(async () => { fireEvent.click(toggle) })
+    expect(localStorage.getItem('theme-mode')).toBe('system')
+
+    await act(async () => { fireEvent.click(toggle) })
+    expect(localStorage.getItem('theme-mode')).toBe('light')
+
+    await act(async () => { fireEvent.click(toggle) })
+    expect(localStorage.getItem('theme-mode')).toBe('dark')
+  })
+})
diff --git a/frontend/src/components/layout/TopNav.tsx b/frontend/src/components/layout/TopNav.tsx
index 35c9dd27..0b0fda9f 100644
--- a/frontend/src/components/layout/TopNav.tsx
+++ b/frontend/src/components/layout/TopNav.tsx
@@ -8,6 +8,8 @@ import { NotificationCenter } from '../notifications/NotificationCenter'
 import { SUPPORTED_LANGUAGES } from '../../i18n/options'
 import type { SupportedLanguage } from '../../i18n/options'
 import './TopNav.css'
+import useTheme from '../../hooks/useTheme'
+import { Sun, Moon, Monitor } from 'lucide-react'
 
 interface TopNavProps {
   onMenuClick: () => void
@@ -45,6 +47,7 @@ const TopNav: React.FC<TopNavProps> = ({
   const bellButtonRef = useRef<HTMLButtonElement>(null)
   const { t, i18n } = useTranslation()
   const location = useLocation()
+  const { mode, setMode } = useTheme()
 
   const activeLanguage = (i18n.resolvedLanguage ?? 'en') as SupportedLanguage
 
@@ -130,6 +133,21 @@ const TopNav: React.FC<TopNavProps> = ({
           />
         </div>
 
+        {/* Theme toggle: cycles light -> dark -> system */}
+        <button
+          className="theme-toggle"
+          onClick={() => {
+            const next: Record<string, 'light' | 'dark' | 'system'> = { light: 'dark', dark: 'system', system: 'light' }
+            setMode(next[mode])
+          }}
+          role="switch"
+          aria-label={mode === 'light' ? 'Switch to dark or system theme' : mode === 'dark' ? 'Switch to system or light theme' : 'Switch to light or dark theme'}
+          aria-checked={mode === 'dark' ? 'true' : mode === 'light' ? 'false' : 'mixed'}
+          title={`Theme: ${mode}`}
+        >
+          {mode === 'light' ? <Sun size={16} /> : mode === 'dark' ? <Moon size={16} /> : <Monitor size={16} />}
+        </button>
+
         <div
           className="language-switcher"
           id="language-switcher"
@@ -155,7 +173,6 @@ const TopNav: React.FC<TopNavProps> = ({
             </button>
           ))}
         </div>
-
         {connected && publicKey ? (
           ready ? (
             <>
diff --git a/frontend/src/context/ThemeContext.tsx b/frontend/src/context/ThemeContext.tsx
new file mode 100644
index 00000000..73a71cc6
--- /dev/null
+++ b/frontend/src/context/ThemeContext.tsx
@@ -0,0 +1,77 @@
+import React, { createContext, useEffect, useState } from 'react'
+
+export type ThemeMode = 'light' | 'dark' | 'system'
+
+interface ThemeContextValue {
+  mode: ThemeMode
+  setMode: (mode: ThemeMode) => void
+  effectiveTheme: 'light' | 'dark'
+}
+
+const ThemeContext = createContext<ThemeContextValue>({
+  mode: 'dark',
+  setMode: () => {},
+  effectiveTheme: 'dark',
+})
+
+export const ThemeProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
+  const [mode, setMode] = useState<ThemeMode>(() => {
+    try {
+      const stored = localStorage.getItem('theme-mode')
+      if (stored === 'light' || stored === 'dark' || stored === 'system') return stored
+    } catch (e) {
+      // ignore
+    }
+    return 'dark'
+  })
+
+  const [systemPrefersDark, setSystemPrefersDark] = useState<boolean>(() => {
+    if (typeof window === 'undefined' || !window.matchMedia) return true
+    return window.matchMedia('(prefers-color-scheme: dark)').matches
+  })
+
+  // Apply theme class to root and persist preference
+  useEffect(() => {
+    const effective = mode === 'system' ? (systemPrefersDark ? 'dark' : 'light') : mode
+    const root = document.documentElement
+    if (effective === 'light') root.classList.add('theme-light')
+    else root.classList.remove('theme-light')
+
+    try {
+      localStorage.setItem('theme-mode', mode)
+    } catch (e) {
+      // ignore
+    }
+  }, [mode, systemPrefersDark])
+
+  // Listen to system preference changes
+  useEffect(() => {
+    if (typeof window === 'undefined' || !window.matchMedia) return
+    const mql = window.matchMedia('(prefers-color-scheme: dark)')
+    const handler = (e: MediaQueryListEvent) => setSystemPrefersDark(e.matches)
+
+    if (typeof mql.addEventListener === 'function') {
+      mql.addEventListener('change', handler)
+    } else if (typeof (mql as any).addListener === 'function') {
+      ;(mql as any).addListener(handler)
+    }
+
+    return () => {
+      if (typeof mql.removeEventListener === 'function') {
+        mql.removeEventListener('change', handler)
+      } else if (typeof (mql as any).removeListener === 'function') {
+        ;(mql as any).removeListener(handler)
+      }
+    }
+  }, [])
+
+  const effectiveTheme = mode === 'system' ? (systemPrefersDark ? 'dark' : 'light') : mode
+
+  return (
+    <ThemeContext.Provider value={{ mode, setMode, effectiveTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export default ThemeContext
diff --git a/frontend/src/hooks/useTheme.ts b/frontend/src/hooks/useTheme.ts
new file mode 100644
index 00000000..ca877fbf
--- /dev/null
+++ b/frontend/src/hooks/useTheme.ts
@@ -0,0 +1,6 @@
+import { useContext } from 'react'
+import ThemeContext from '../context/ThemeContext'
+
+export const useTheme = () => useContext(ThemeContext)
+
+export default useTheme
diff --git a/frontend/src/styles/global.css b/frontend/src/styles/global.css
index 0903171a..73998584 100644
--- a/frontend/src/styles/global.css
+++ b/frontend/src/styles/global.css
@@ -201,3 +201,36 @@ input:focus, textarea:focus {
     padding: 12px 16px;
   }
 }
+
+/* Theme transition helpers */
+body,
+.glass-panel,
+input,
+textarea,
+th,
+td,
+.dag-node,
+.chip {
+  transition: background-color 300ms ease, color 300ms ease, border-color 300ms ease, box-shadow 300ms ease;
+}
+
+/* Light theme overrides */
+.theme-light {
+  --bg-primary: #FFFFFF;
+  --bg-surface: #F8FAFC;
+  --bg-surface-alt: #F1F5F9;
+  --bg-secondary: #F3F4F6;
+  --border-color: #E6E9EE;
+  --border-subtle: #E9EEF4;
+  --text-primary: #0A0E14;
+  --text-secondary: #475569;
+  --accent-cyan: #0EA5E9;
+  --accent-purple: #7C3AED;
+  --accent-green: #059669;
+  --panel-bg: var(--bg-surface);
+  --panel-border: var(--border-color);
+  --primary: var(--accent-purple);
+  --primary-hover: #6d28d9;
+  --accent: var(--accent-cyan);
+}
+