// tailwind.config.js
module.exports = {
  darkMode: 'class', // or 'media' for system-only
  theme: {
    extend: {
      colors: {
        // Semantic color tokens
        background: 'hsl(var(--background))',
        foreground: 'hsl(var(--foreground))',
        primary: 'hsl(var(--primary))',
        muted: 'hsl(var(--muted))',
      },
    },
  },
};

/* globals.css */
@tailwind base;
@tailwind components;
@tailwind utilities;

@layer base {
  :root {
    --background: 0 0% 100%;
    --foreground: 222 47% 11%;
    --primary: 221 83% 53%;
    --primary-foreground: 210 40% 98%;
    --secondary: 210 40% 96%;
    --secondary-foreground: 222 47% 11%;
    --muted: 210 40% 96%;
    --muted-foreground: 215 16% 47%;
    --accent: 210 40% 96%;
    --accent-foreground: 222 47% 11%;
    --destructive: 0 84% 60%;
    --destructive-foreground: 210 40% 98%;
    --border: 214 32% 91%;
    --input: 214 32% 91%;
    --ring: 221 83% 53%;
    --radius: 0.5rem;
  }

  .dark {
    --background: 222 47% 11%;
    --foreground: 210 40% 98%;
    --primary: 217 91% 60%;
    --primary-foreground: 222 47% 11%;
    --secondary: 217 33% 17%;
    --secondary-foreground: 210 40% 98%;
    --muted: 217 33% 17%;
    --muted-foreground: 215 20% 65%;
    --accent: 217 33% 17%;
    --accent-foreground: 210 40% 98%;
    --destructive: 0 62% 30%;
    --destructive-foreground: 210 40% 98%;
    --border: 217 33% 17%;
    --input: 217 33% 17%;
    --ring: 224 76% 48%;
  }
}

@layer base {
  * {
    @apply border-border;
  }
  body {
    @apply bg-background text-foreground;
  }
}

<!-- Component with dark mode variants -->
<div class="bg-white dark:bg-gray-900">
  <h1 class="text-gray-900 dark:text-white">Title</h1>
  <p class="text-gray-600 dark:text-gray-300">Description</p>
  <button class="bg-blue-500 hover:bg-blue-600 dark:bg-blue-600 dark:hover:bg-blue-700">
    Action
  </button>
</div>

// lib/theme-context.tsx
'use client';

import { createContext, useContext, useEffect, useState } from 'react';

type Theme = 'light' | 'dark' | 'system';

interface ThemeContextType {
  theme: Theme;
  setTheme: (theme: Theme) => void;
  resolvedTheme: 'light' | 'dark';
}

const ThemeContext = createContext<ThemeContextType | undefined>(undefined);

const STORAGE_KEY = 'theme';

export function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setThemeState] = useState<Theme>('system');
  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');
  const [mounted, setMounted] = useState(false);

  // Get system preference
  const getSystemTheme = (): 'light' | 'dark' => {
    if (typeof window === 'undefined') return 'light';
    return window.matchMedia('(prefers-color-scheme: dark)').matches
      ? 'dark'
      : 'light';
  };

  // Apply theme to document
  const applyTheme = (theme: Theme) => {
    const root = document.documentElement;
    const resolved = theme === 'system' ? getSystemTheme() : theme;

    root.classList.remove('light', 'dark');
    root.classList.add(resolved);
    setResolvedTheme(resolved);
  };

  // Set theme and persist
  const setTheme = (newTheme: Theme) => {
    setThemeState(newTheme);
    localStorage.setItem(STORAGE_KEY, newTheme);
    applyTheme(newTheme);
  };

  // Initialize theme on mount
  useEffect(() => {
    const stored = localStorage.getItem(STORAGE_KEY) as Theme | null;
    const initialTheme = stored || 'system';
    setThemeState(initialTheme);
    applyTheme(initialTheme);
    setMounted(true);
  }, []);

  // Listen for system preference changes
  useEffect(() => {
    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');

    const handleChange = () => {
      if (theme === 'system') {
        applyTheme('system');
      }
    };

    mediaQuery.addEventListener('change', handleChange);
    return () => mediaQuery.removeEventListener('change', handleChange);
  }, [theme]);

  // Prevent hydration mismatch
  if (!mounted) {
    return <>{children}</>;
  }

  return (
    <ThemeContext.Provider value={{ theme, setTheme, resolvedTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

export function useTheme() {
  const context = useContext(ThemeContext);
  if (!context) {
    throw new Error('useTheme must be used within a ThemeProvider');
  }
  return context;
}

// app/layout.tsx
import { ThemeProvider } from '@/lib/theme-context';

// Inline script to prevent flash
const themeScript = `
  (function() {
    const stored = localStorage.getItem('theme');
    const theme = stored || 'system';
    const resolved = theme === 'system'
      ? window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
      : theme;
    document.documentElement.classList.add(resolved);
  })();
`;

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" suppressHydrationWarning>
      <head>
        <script dangerouslySetInnerHTML={{ __html: themeScript }} />
      </head>
      <body>
        <ThemeProvider>{children}</ThemeProvider>
      </body>
    </html>
  );
}

// components/ThemeToggle.tsx
'use client';

import { useTheme } from '@/lib/theme-context';
import { Moon, Sun } from 'lucide-react';

export function ThemeToggle() {
  const { resolvedTheme, setTheme } = useTheme();

  return (
    <button
      onClick={() => setTheme(resolvedTheme === 'dark' ? 'light' : 'dark')}
      className="rounded-lg p-2 hover:bg-gray-100 dark:hover:bg-gray-800"
      aria-label={`Switch to ${resolvedTheme === 'dark' ? 'light' : 'dark'} mode`}
    >
      {resolvedTheme === 'dark' ? (
        <Sun className="h-5 w-5" />
      ) : (
        <Moon className="h-5 w-5" />
      )}
    </button>
  );
}

// components/ThemeSelector.tsx
'use client';

import { useTheme } from '@/lib/theme-context';
import { Monitor, Moon, Sun } from 'lucide-react';

const themes = [
  { value: 'light', icon: Sun, label: 'Light' },
  { value: 'dark', icon: Moon, label: 'Dark' },
  { value: 'system', icon: Monitor, label: 'System' },
] as const;

export function ThemeSelector() {
  const { theme, setTheme } = useTheme();

  return (
    <div className="flex rounded-lg bg-gray-100 p-1 dark:bg-gray-800">
      {themes.map(({ value, icon: Icon, label }) => (
        <button
          key={value}
          onClick={() => setTheme(value)}
          className={`
            flex items-center gap-2 rounded-md px-3 py-1.5 text-sm font-medium
            transition-colors
            ${theme === value
              ? 'bg-white text-gray-900 shadow dark:bg-gray-700 dark:text-white'
              : 'text-gray-600 hover:text-gray-900 dark:text-gray-400 dark:hover:text-white'
            }
          `}
          aria-label={`Switch to ${label} theme`}
        >
          <Icon className="h-4 w-4" />
          <span className="hidden sm:inline">{label}</span>
        </button>
      ))}
    </div>
  );
}

// components/ThemeSwitch.tsx
'use client';

import { useTheme } from '@/lib/theme-context';
import { motion } from 'framer-motion';

export function ThemeSwitch() {
  const { resolvedTheme, setTheme } = useTheme();
  const isDark = resolvedTheme === 'dark';

  return (
    <button
      onClick={() => setTheme(isDark ? 'light' : 'dark')}
      className="relative h-8 w-14 rounded-full bg-gray-200 p-1 dark:bg-gray-700"
      aria-label={`Switch to ${isDark ? 'light' : 'dark'} mode`}
    >
      <motion.div
        className="flex h-6 w-6 items-center justify-center rounded-full bg-white shadow-md"
        animate={{ x: isDark ? 24 : 0 }}
        transition={{ type: 'spring', stiffness: 500, damping: 30 }}
      >
        <motion.span
          initial={false}
          animate={{ rotate: isDark ? 360 : 0 }}
          transition={{ duration: 0.5 }}
        >
          {isDark ? '🌙' : '☀️'}
        </motion.span>
      </motion.div>
    </button>
  );
}

/* For simple sites without JavaScript */
:root {
  --bg: #ffffff;
  --text: #1a1a1a;
  --primary: #3b82f6;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0a0a0a;
    --text: #fafafa;
    --primary: #60a5fa;
  }
}

body {
  background-color: var(--bg);
  color: var(--text);
}

:root {
  /* Background colors */
  --color-bg-primary: #ffffff;
  --color-bg-secondary: #f9fafb;
  --color-bg-tertiary: #f3f4f6;
  --color-bg-inverse: #111827;

  /* Text colors */
  --color-text-primary: #111827;
  --color-text-secondary: #4b5563;
  --color-text-tertiary: #9ca3af;
  --color-text-inverse: #ffffff;

  /* Border colors */
  --color-border-primary: #e5e7eb;
  --color-border-secondary: #d1d5db;

  /* Interactive colors */
  --color-interactive-primary: #3b82f6;
  --color-interactive-hover: #2563eb;
  --color-interactive-active: #1d4ed8;

  /* Status colors */
  --color-success: #10b981;
  --color-warning: #f59e0b;
  --color-error: #ef4444;
  --color-info: #3b82f6;
}

.dark {
  --color-bg-primary: #0f172a;
  --color-bg-secondary: #1e293b;
  --color-bg-tertiary: #334155;
  --color-bg-inverse: #f8fafc;

  --color-text-primary: #f8fafc;
  --color-text-secondary: #cbd5e1;
  --color-text-tertiary: #64748b;
  --color-text-inverse: #0f172a;

  --color-border-primary: #334155;
  --color-border-secondary: #475569;

  --color-interactive-primary: #60a5fa;
  --color-interactive-hover: #3b82f6;
  --color-interactive-active: #2563eb;

  --color-success: #34d399;
  --color-warning: #fbbf24;
  --color-error: #f87171;
  --color-info: #60a5fa;
}

npm install next-themes

// app/providers.tsx
'use client';

import { ThemeProvider } from 'next-themes';

export function Providers({ children }: { children: React.ReactNode }) {
  return (
    <ThemeProvider attribute="class" defaultTheme="system" enableSystem>
      {children}
    </ThemeProvider>
  );
}

// app/layout.tsx
import { Providers } from './providers';

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" suppressHydrationWarning>
      <body>
        <Providers>{children}</Providers>
      </body>
    </html>
  );
}

// components/ThemeToggle.tsx
'use client';

import { useTheme } from 'next-themes';
import { useEffect, useState } from 'react';

export function ThemeToggle() {
  const { theme, setTheme } = useTheme();
  const [mounted, setMounted] = useState(false);

  useEffect(() => setMounted(true), []);

  if (!mounted) return null;

  return (
    <select value={theme} onChange={(e) => setTheme(e.target.value)}>
      <option value="system">System</option>
      <option value="light">Light</option>
      <option value="dark">Dark</option>
    </select>
  );
}

// Swap images based on theme
<picture>
  <source srcSet="/dark-logo.svg" media="(prefers-color-scheme: dark)" />
  <img src="/light-logo.svg" alt="Logo" />
</picture>

// With Tailwind
<img src="/light-logo.svg" alt="Logo" class="dark:hidden" />
<img src="/dark-logo.svg" alt="Logo" class="hidden dark:block" />

// SVG that adapts to theme
<svg className="text-gray-900 dark:text-white" fill="currentColor">
  {/* ... */}
</svg>

// tests/dark-mode.spec.ts
import { test, expect } from '@playwright/test';

test.describe('Dark Mode', () => {
  test('respects system preference', async ({ page }) => {
    await page.emulateMedia({ colorScheme: 'dark' });
    await page.goto('/');

    const html = page.locator('html');
    await expect(html).toHaveClass(/dark/);
  });

  test('toggles theme correctly', async ({ page }) => {
    await page.goto('/');

    await page.click('[aria-label*="dark"]');
    await expect(page.locator('html')).toHaveClass(/dark/);

    await page.click('[aria-label*="light"]');
    await expect(page.locator('html')).not.toHaveClass(/dark/);
  });

  test('persists theme preference', async ({ page }) => {
    await page.goto('/');
    await page.click('[aria-label*="dark"]');

    await page.reload();
    await expect(page.locator('html')).toHaveClass(/dark/);
  });
});