yude.jp/pages/components/themeContext.js

import React from 'react'

const getInitialTheme = _ => {
    if (typeof window !== "undefined" && window.localStorage) {
      const storedPrefs = window.localStorage.getItem("color-theme")
      if (typeof storedPrefs === "string") {
        return storedPrefs
      }
  
      const userMedia = window.matchMedia("(prefers-color-scheme: dark)")
      if (userMedia.matches) {
        return "dark"
      }
    }
  
    // If you want to use light theme as the default, return "light" instead
    return "dark"
  }

const ThemeContext = React.createContext()

export default ThemeContext

export const ThemeProvider = ({ initialTheme, children }) => {
    const [theme, setTheme] = React.useState(getInitialTheme)
  
    const rawSetTheme = theme => {
      const root = window.document.documentElement
      const isDark = theme === "dark"
  
      root.classList.remove(isDark ? "light" : "dark")
      root.classList.add(theme)
  
      localStorage.setItem("color-theme", theme)
    }
  
    if (initialTheme) {
      rawSetTheme(initialTheme)
    }
  
    React.useEffect(
      _ => {
        rawSetTheme(theme)
      },
      [theme]
    )
  
    return (
      <ThemeContext.Provider value={{ theme, setTheme }}>
        {children}
      </ThemeContext.Provider>
    )
  }