Dark mode with Tailwind and localStorage persistence

import React, { createContext, useContext, useEffect, useState, ReactNode } from 'react'

type Theme = 'light' | 'dark'

interface ThemeContextType {
  theme: Theme
  toggleTheme: () => void
}

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

export function ThemeProvider({ children }: { children: ReactNode }) {
  const [theme, setTheme] = useState<Theme>(() => {
    const stored = localStorage.getItem('theme') as Theme | null
    if (stored) return stored

    // Detect system preference
    if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
      return 'dark'
    }
    return 'light'
  })

  useEffect(() => {
    const root = document.documentElement
    root.classList.remove('light', 'dark')
    root.classList.add(theme)
    localStorage.setItem('theme', theme)
  }, [theme])

  // Sync across tabs
  useEffect(() => {
    const handleStorage = (e: StorageEvent) => {
      if (e.key === 'theme' && e.newValue) {
        setTheme(e.newValue as Theme)
      }
    }

    window.addEventListener('storage', handleStorage)
    return () => window.removeEventListener('storage', handleStorage)
  }, [])

  const toggleTheme = () => {
    setTheme((prev) => (prev === 'light' ? 'dark' : 'light'))
  }

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

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