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frontend/src/hooks/use-theme.js

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+import { useState, useEffect } from 'react';
+
+export function useTheme() {
+    const [theme, setTheme] = useState(localStorage.getItem('theme') || 'dark');
+
+    useEffect(() => {
+        const root = window.document.documentElement;
+        root.classList.remove('light', 'dark');
+        root.classList.add(theme);
+        localStorage.setItem('theme', theme);
+    }, [theme]);
+
+    const toggleTheme = () => setTheme(prev => prev === 'dark' ? 'light' : 'dark');
+
+    return { theme, toggleTheme };
+}