client/src/hooks/ThemeContext.jsx

//ThemeContext.js
// source: https://plainenglish.io/blog/light-and-dark-mode-in-react-web-application-with-tailwind-css-89674496b942

import React, { createContext, useState, useEffect } 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';
    }
  }

  return 'light'; // light theme as the default;
};

export const ThemeContext = createContext();

export const ThemeProvider = ({ initialTheme, children }) => {
  const [theme, setTheme] = useState(getInitialTheme);

  const rawSetTheme = (rawTheme) => {
    const root = window.document.documentElement;
    let isDark = rawTheme === 'dark';

    if (rawTheme === 'system') {
      isDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
    }

    root.classList.remove(isDark ? 'light' : 'dark');
    root.classList.add(isDark ? 'dark' : 'light');

    localStorage.setItem('color-theme', rawTheme);
  };

  if (initialTheme) {
    rawSetTheme(initialTheme);
  }

  useEffect(() => {
    rawSetTheme(theme);
  }, [theme]);

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