task-manager/client/src/components/ThemeToggle.tsx

import { useState, useEffect } from 'react';
import { Button } from '@/components/ui/button';
import { Moon, Sun } from 'lucide-react';

export default function ThemeToggle() {
  const [isDark, setIsDark] = useState(false);

  useEffect(() => {
    // Check for saved theme preference or default to light mode
    const savedTheme = localStorage.getItem('theme');
    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
    
    const shouldBeDark = savedTheme === 'dark' || (!savedTheme && prefersDark);
    setIsDark(shouldBeDark);
    
    // Apply theme to document
    if (shouldBeDark) {
      document.documentElement.classList.add('dark');
    } else {
      document.documentElement.classList.remove('dark');
    }
  }, []);

  const toggleTheme = () => {
    const newTheme = !isDark;
    setIsDark(newTheme);
    
    if (newTheme) {
      document.documentElement.classList.add('dark');
      localStorage.setItem('theme', 'dark');
    } else {
      document.documentElement.classList.remove('dark');
      localStorage.setItem('theme', 'light');
    }
    
    console.log('Theme changed to:', newTheme ? 'dark' : 'light');
  };

  return (
    <Button 
      variant="ghost" 
      size="icon"
      onClick={toggleTheme}
      data-testid="button-theme-toggle"
      className="w-9 h-9"
    >
      {isDark ? (
        <Sun className="w-4 h-4" />
      ) : (
        <Moon className="w-4 h-4" />
      )}
    </Button>
  );
}