diff --git a/webui/src/App.tsx b/webui/src/App.tsx
index a909ccb..44a155b 100644
--- a/webui/src/App.tsx
+++ b/webui/src/App.tsx
@@ -7,6 +7,7 @@ import SystemInfoDialog from "./components/SystemInfoDialog";
 import { type CreateInstanceOptions, type Instance } from "@/types/instance";
 import { useInstances } from "@/contexts/InstancesContext";
 import { useAuth } from "@/contexts/AuthContext";
+import { ThemeProvider } from "@/contexts/ThemeContext";
 
 function App() {
   const { isAuthenticated, isLoading: authLoading } = useAuth();
@@ -42,44 +43,50 @@ function App() {
   // Show loading spinner while checking auth
   if (authLoading) {
     return (
-      <div className="min-h-screen bg-gray-50 flex items-center justify-center">
-        <div className="text-center">
-          <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-blue-600 mx-auto mb-4"></div>
-          <p className="text-gray-600">Loading...</p>
+      <ThemeProvider>
+        <div className="min-h-screen bg-background flex items-center justify-center">
+          <div className="text-center">
+            <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-primary mx-auto mb-4"></div>
+            <p className="text-muted-foreground">Loading...</p>
+          </div>
         </div>
-      </div>
+      </ThemeProvider>
     );
   }
 
   // Show login dialog if not authenticated
   if (!isAuthenticated) {
     return (
-      <div className="min-h-screen bg-gray-50">
-        <LoginDialog open={true} />
-      </div>
+      <ThemeProvider>
+        <div className="min-h-screen bg-background">
+          <LoginDialog open={true} />
+        </div>
+      </ThemeProvider>
     );
   }
 
   // Show main app if authenticated
   return (
-    <div className="min-h-screen bg-gray-50">
-      <Header onCreateInstance={handleCreateInstance} onShowSystemInfo={handleShowSystemInfo} />
-      <main className="container mx-auto max-w-4xl px-4 py-8">
-        <InstanceList editInstance={handleEditInstance} />
-      </main>
+    <ThemeProvider>
+      <div className="min-h-screen bg-background">
+        <Header onCreateInstance={handleCreateInstance} onShowSystemInfo={handleShowSystemInfo} />
+        <main className="container mx-auto max-w-4xl px-4 py-8">
+          <InstanceList editInstance={handleEditInstance} />
+        </main>
 
-      <InstanceDialog
-        open={isInstanceModalOpen}
-        onOpenChange={setIsInstanceModalOpen}
-        onSave={handleSaveInstance}
-        instance={editingInstance}
-      />
+        <InstanceDialog
+          open={isInstanceModalOpen}
+          onOpenChange={setIsInstanceModalOpen}
+          onSave={handleSaveInstance}
+          instance={editingInstance}
+        />
 
-      <SystemInfoDialog
-        open={isSystemInfoModalOpen}
-        onOpenChange={setIsSystemInfoModalOpen}
-      />
-    </div>
+        <SystemInfoDialog
+          open={isSystemInfoModalOpen}
+          onOpenChange={setIsSystemInfoModalOpen}
+        />
+      </div>
+    </ThemeProvider>
   );
 }
 
diff --git a/webui/src/__tests__/App.test.tsx b/webui/src/__tests__/App.test.tsx
index 1d22531..04c1e34 100644
--- a/webui/src/__tests__/App.test.tsx
+++ b/webui/src/__tests__/App.test.tsx
@@ -55,6 +55,21 @@ describe('App Component - Critical Business Logic Only', () => {
     vi.mocked(instancesApi.list).mockResolvedValue(mockInstances)
     window.sessionStorage.setItem('llamactl_management_key', 'test-api-key-123')
     global.fetch = vi.fn(() => Promise.resolve(new Response(null, { status: 200 })))
+    
+    // Mock window.matchMedia for dark mode functionality
+    Object.defineProperty(window, 'matchMedia', {
+      writable: true,
+      value: vi.fn().mockImplementation((query: string) => ({
+        matches: false,
+        media: query,
+        onchange: null,
+        addListener: vi.fn(),
+        removeListener: vi.fn(),
+        addEventListener: vi.fn(),
+        removeEventListener: vi.fn(),
+        dispatchEvent: vi.fn(),
+      })),
+    })
   })
 
   afterEach(() => {
diff --git a/webui/src/components/Header.tsx b/webui/src/components/Header.tsx
index ed272ed..3c7e0e1 100644
--- a/webui/src/components/Header.tsx
+++ b/webui/src/components/Header.tsx
@@ -1,6 +1,7 @@
 import { Button } from "@/components/ui/button";
-import { HelpCircle, LogOut } from "lucide-react";
+import { HelpCircle, LogOut, Moon, Sun } from "lucide-react";
 import { useAuth } from "@/contexts/AuthContext";
+import { useTheme } from "@/contexts/ThemeContext";
 
 interface HeaderProps {
   onCreateInstance: () => void;
@@ -9,6 +10,7 @@ interface HeaderProps {
 
 function Header({ onCreateInstance, onShowSystemInfo }: HeaderProps) {
   const { logout } = useAuth();
+  const { theme, toggleTheme } = useTheme();
 
   const handleLogout = () => {
     if (confirm("Are you sure you want to logout?")) {
@@ -17,10 +19,10 @@ function Header({ onCreateInstance, onShowSystemInfo }: HeaderProps) {
   };
 
   return (
-    <header className="bg-white border-b border-gray-200">
+    <header className="bg-card border-b border-border">
       <div className="container mx-auto max-w-4xl px-4 py-4">
         <div className="flex items-center justify-between">
-          <h1 className="text-2xl font-bold text-gray-900">
+          <h1 className="text-2xl font-bold text-foreground">
             Llamactl Dashboard
           </h1>
 
@@ -29,6 +31,16 @@ function Header({ onCreateInstance, onShowSystemInfo }: HeaderProps) {
               Create Instance
             </Button>
 
+            <Button
+              variant="outline"
+              size="icon"
+              onClick={toggleTheme}
+              data-testid="theme-toggle-button"
+              title={`Switch to ${theme === 'light' ? 'dark' : 'light'} mode`}
+            >
+              {theme === 'light' ? <Moon className="h-4 w-4" /> : <Sun className="h-4 w-4" />}
+            </Button>
+
             <Button
               variant="outline"
               size="icon"
diff --git a/webui/src/components/InstanceList.tsx b/webui/src/components/InstanceList.tsx
index 39c924d..fd258c5 100644
--- a/webui/src/components/InstanceList.tsx
+++ b/webui/src/components/InstanceList.tsx
@@ -18,8 +18,8 @@ function InstanceList({ editInstance }: InstanceListProps) {
     return (
       <div className="flex items-center justify-center py-12" aria-label="Loading">
         <div className="text-center">
-          <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-blue-600 mx-auto mb-4"></div>
-          <p className="text-gray-600">Loading instances...</p>
+          <div className="animate-spin rounded-full h-8 w-8 border-b-2 border-primary mx-auto mb-4"></div>
+          <p className="text-muted-foreground">Loading instances...</p>
         </div>
       </div>
     )
@@ -28,7 +28,7 @@ function InstanceList({ editInstance }: InstanceListProps) {
   if (error) {
     return (
       <div className="text-center py-12">
-        <div className="text-red-600 mb-4">
+        <div className="text-destructive mb-4">
           <p className="text-lg font-semibold">Error loading instances</p>
           <p className="text-sm">{error}</p>
         </div>
@@ -39,15 +39,15 @@ function InstanceList({ editInstance }: InstanceListProps) {
   if (instances.length === 0) {
     return (
       <div className="text-center py-12">
-        <p className="text-gray-600 text-lg mb-2">No instances found</p>
-        <p className="text-gray-500 text-sm">Create your first instance to get started</p>
+        <p className="text-foreground text-lg mb-2">No instances found</p>
+        <p className="text-muted-foreground text-sm">Create your first instance to get started</p>
       </div>
     )
   }
 
   return (
     <div className="space-y-4">
-      <h2 className="text-xl font-semibold text-gray-900 mb-6">
+      <h2 className="text-xl font-semibold text-foreground mb-6">
         Instances ({instances.length})
       </h2>
       
diff --git a/webui/src/contexts/ThemeContext.tsx b/webui/src/contexts/ThemeContext.tsx
new file mode 100644
index 0000000..d05d49a
--- /dev/null
+++ b/webui/src/contexts/ThemeContext.tsx
@@ -0,0 +1,54 @@
+import { createContext, useContext, useEffect, useState, type ReactNode } from "react";
+
+type Theme = "light" | "dark";
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme: () => void;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+interface ThemeProviderProps {
+  children: ReactNode;
+}
+
+export function ThemeProvider({ children }: ThemeProviderProps) {
+  const [theme, setTheme] = useState<Theme>(() => {
+    const stored = localStorage.getItem("theme");
+    if (stored === "light" || stored === "dark") {
+      return stored;
+    }
+    return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+  });
+
+  useEffect(() => {
+    const root = document.documentElement;
+    
+    if (theme === "dark") {
+      root.classList.add("dark");
+    } else {
+      root.classList.remove("dark");
+    }
+    
+    localStorage.setItem("theme", theme);
+  }, [theme]);
+
+  const toggleTheme = () => {
+    setTheme(prevTheme => prevTheme === "light" ? "dark" : "light");
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (context === undefined) {
+    throw new Error("useTheme must be used within a ThemeProvider");
+  }
+  return context;
+}
\ No newline at end of file