diff --git a/package-lock.json b/package-lock.json
index da62c53..30d1f84 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -39,6 +39,7 @@
         "genkit": "^1.8.0",
         "lucide-react": "^0.475.0",
         "next": "15.3.3",
+        "next-themes": "^0.3.0",
         "patch-package": "^8.0.0",
         "react": "^18.3.1",
         "react-day-picker": "^8.10.1",
@@ -7653,6 +7654,16 @@
         }
       }
     },
+    "node_modules/next-themes": {
+      "version": "0.3.0",
+      "resolved": "https://registry.npmjs.org/next-themes/-/next-themes-0.3.0.tgz",
+      "integrity": "sha512-/QHIrsYpd6Kfk7xakK4svpDI5mmXP0gfvCoJdGpZQ2TOrQZmsW0QxjaiLn8wbIKjtm4BTSqLoix4lxYYOnLJ/w==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^16.8 || ^17 || ^18",
+        "react-dom": "^16.8 || ^17 || ^18"
+      }
+    },
     "node_modules/next/node_modules/postcss": {
       "version": "8.4.31",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.4.31.tgz",
diff --git a/package.json b/package.json
index 914649d..509cf85 100644
--- a/package.json
+++ b/package.json
@@ -43,6 +43,7 @@
     "genkit": "^1.8.0",
     "lucide-react": "^0.475.0",
     "next": "15.3.3",
+    "next-themes": "^0.3.0",
     "patch-package": "^8.0.0",
     "react": "^18.3.1",
     "react-day-picker": "^8.10.1",
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index b037e71..b86307b 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -5,6 +5,7 @@ import { Toaster } from "@/components/ui/toaster";
 import Header from '@/components/layout/Header';
 import Footer from '@/components/layout/Footer';
 import { cn } from '@/lib/utils';
+import { ThemeProvider } from '@/components/theme-provider';
 
 const ptSans = PT_Sans({
   subsets: ['latin'],
@@ -36,12 +37,19 @@ export default function RootLayout({
         className={cn('min-h-screen bg-background font-body antialiased flex flex-col', ptSans.variable)}
         suppressHydrationWarning={true}
       >
-        <Header />
-        <main className="flex-grow container mx-auto px-4 py-8">
-          {children}
-        </main>
-        <Footer />
-        <Toaster />
+        <ThemeProvider
+          attribute="class"
+          defaultTheme="system"
+          enableSystem
+          disableTransitionOnChange
+        >
+          <Header />
+          <main className="flex-grow container mx-auto px-4 py-8">
+            {children}
+          </main>
+          <Footer />
+          <Toaster />
+        </ThemeProvider>
       </body>
     </html>
   );
diff --git a/src/components/layout/Header.tsx b/src/components/layout/Header.tsx
index 3b97297..db8718f 100644
--- a/src/components/layout/Header.tsx
+++ b/src/components/layout/Header.tsx
@@ -13,25 +13,21 @@ import {
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
 import { useState, useEffect } from 'react';
+import { ThemeToggleButton } from '@/components/ui/theme-toggle';
 
 export default function Header() {
   const pathname = usePathname();
-  // Mock authentication state
   const [isAuthenticated, setIsAuthenticated] = useState(false);
-  // This effect runs only on the client after hydration
+  
   useEffect(() => {
-    // In a real app, you'd check localStorage or an auth context
-    // For now, we'll simulate it.
-    // To test logged-in state, you can manually set this in browser console: localStorage.setItem('isToyShareAuthenticated', 'true');
     const authStatus = localStorage.getItem('isToyShareAuthenticated');
     setIsAuthenticated(authStatus === 'true');
-  }, [pathname]); // Re-check on path change, e.g. after login/logout actions
+  }, [pathname]);
 
   const handleLogout = () => {
     localStorage.removeItem('isToyShareAuthenticated');
     setIsAuthenticated(false);
-    // Potentially redirect to home or login page
-    // router.push('/'); // if using useRouter
+    // router.push('/'); // Consider redirecting if not already handled by auth checks elsewhere
   };
 
 
@@ -42,58 +38,62 @@ export default function Header() {
           <ToyBrick className="h-7 w-7" />
           <h1 className="text-2xl font-headline font-bold">ToyShare</h1>
         </Link>
-        <nav className="flex items-center gap-4">
-          <Link href="/" passHref>
-            <Button variant={pathname === '/' ? 'secondary' : 'ghost'} size="sm">
-              Browse Toys
-            </Button>
-          </Link>
-          {isAuthenticated ? (
-            <DropdownMenu>
-              <DropdownMenuTrigger asChild>
-                <Button variant="ghost" size="icon" className="rounded-full">
-                  <UserCircle2 className="h-6 w-6" />
-                </Button>
-              </DropdownMenuTrigger>
-              <DropdownMenuContent align="end">
-                <DropdownMenuLabel>My Account</DropdownMenuLabel>
-                <DropdownMenuSeparator />
-                <DropdownMenuItem asChild>
-                  <Link href="/dashboard">
-                    <LayoutDashboard className="mr-2 h-4 w-4" />
-                    Dashboard
-                  </Link>
-                </DropdownMenuItem>
-                <DropdownMenuItem asChild>
-                   <Link href="/dashboard/profile">
-                    <UserCircle2 className="mr-2 h-4 w-4" />
-                    Profile
-                  </Link>
-                </DropdownMenuItem>
-                <DropdownMenuSeparator />
-                <DropdownMenuItem onClick={handleLogout}>
-                  <LogIn className="mr-2 h-4 w-4" />
-                  Logout
-                </DropdownMenuItem>
-              </DropdownMenuContent>
-            </DropdownMenu>
-          ) : (
-            <>
-              <Link href="/login" passHref>
-                <Button variant={pathname === '/login' ? 'secondary' : 'ghost'} size="sm">
-                  <LogIn className="mr-2 h-4 w-4" />
-                  Login
-                </Button>
-              </Link>
-              <Link href="/register" passHref>
-                <Button variant="default" size="sm">
-                  <UserPlus className="mr-2 h-4 w-4" />
-                  Register
-                </Button>
-              </Link>
-            </>
-          )}
-        </nav>
+        
+        <div className="flex items-center gap-2">
+          <nav className="flex items-center gap-1 sm:gap-2">
+            <Link href="/" passHref>
+              <Button variant={pathname === '/' ? 'secondary' : 'ghost'} size="sm" className="px-2 sm:px-3">
+                Browse Toys
+              </Button>
+            </Link>
+            {isAuthenticated ? (
+              <DropdownMenu>
+                <DropdownMenuTrigger asChild>
+                  <Button variant="ghost" size="icon" className="rounded-full h-9 w-9 md:h-10 md:w-10">
+                    <UserCircle2 className="h-6 w-6" />
+                  </Button>
+                </DropdownMenuTrigger>
+                <DropdownMenuContent align="end">
+                  <DropdownMenuLabel>My Account</DropdownMenuLabel>
+                  <DropdownMenuSeparator />
+                  <DropdownMenuItem asChild>
+                    <Link href="/dashboard">
+                      <LayoutDashboard className="mr-2 h-4 w-4" />
+                      Dashboard
+                    </Link>
+                  </DropdownMenuItem>
+                  <DropdownMenuItem asChild>
+                     <Link href="/dashboard/profile">
+                      <UserCircle2 className="mr-2 h-4 w-4" />
+                      Profile
+                    </Link>
+                  </DropdownMenuItem>
+                  <DropdownMenuSeparator />
+                  <DropdownMenuItem onClick={handleLogout}>
+                    <LogIn className="mr-2 h-4 w-4" />
+                    Logout
+                  </DropdownMenuItem>
+                </DropdownMenuContent>
+              </DropdownMenu>
+            ) : (
+              <>
+                <Link href="/login" passHref>
+                  <Button variant={pathname === '/login' ? 'secondary' : 'ghost'} size="sm" className="px-2 sm:px-3">
+                    <LogIn className="mr-2 h-4 w-4" />
+                    Login
+                  </Button>
+                </Link>
+                <Link href="/register" passHref>
+                  <Button variant="default" size="sm" className="px-2 sm:px-3">
+                    <UserPlus className="mr-2 h-4 w-4" />
+                    Register
+                  </Button>
+                </Link>
+              </>
+            )}
+          </nav>
+          <ThemeToggleButton />
+        </div>
       </div>
     </header>
   );
diff --git a/src/components/theme-provider.tsx b/src/components/theme-provider.tsx
new file mode 100644
index 0000000..0e31ee8
--- /dev/null
+++ b/src/components/theme-provider.tsx
@@ -0,0 +1,9 @@
+'use client';
+
+import * as React from 'react';
+import { ThemeProvider as NextThemesProvider } from 'next-themes';
+import type { ThemeProviderProps } from 'next-themes/dist/types';
+
+export function ThemeProvider({ children, ...props }: ThemeProviderProps) {
+  return <NextThemesProvider {...props}>{children}</NextThemesProvider>;
+}
diff --git a/src/components/ui/theme-toggle.tsx b/src/components/ui/theme-toggle.tsx
new file mode 100644
index 0000000..c5f4cfe
--- /dev/null
+++ b/src/components/ui/theme-toggle.tsx
@@ -0,0 +1,50 @@
+'use client';
+
+import * as React from 'react';
+import { Moon, Sun } from 'lucide-react';
+import { useTheme } from 'next-themes';
+
+import { Button } from '@/components/ui/button';
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from '@/components/ui/dropdown-menu';
+
+export function ThemeToggleButton() {
+  const { setTheme } = useTheme();
+  const [mounted, setMounted] = React.useState(false);
+
+  React.useEffect(() => {
+    setMounted(true);
+  }, []);
+
+  if (!mounted) {
+    // Render a placeholder or null to avoid hydration mismatch, matching size of actual button
+    return <Button variant="outline" size="icon" disabled className="h-9 w-9 md:h-10 md:w-10 opacity-0 pointer-events-none"><Sun className="h-[1.2rem] w-[1.2rem]" /></Button>;
+  }
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="outline" size="icon" className="h-9 w-9 md:h-10 md:w-10">
+          <Sun className="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme('light')}>
+          Light
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('dark')}>
+          Dark
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('system')}>
+          System
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}