darkmode-6.js

function userTheme(toggle = false) {
  let userMode = localStorage.userThemeMode || 'auto';
  const osMode = !!window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches ? 'dark' : 'light';
  if(toggle) {
   switch(userMode) {
    case 'auto':
     userMode = 'dark'; break;
    case 'dark':
     userMode = 'light'; break;
    default:
     userMode = 'auto';
   }
   localStorage.userThemeMode = userMode;
  }
  console.log(`current mode : ${userMode}`);
  window.__THEME_MODE = userMode === 'auto' ? osMode : userMode;
  document.getElementsByTagName('html')[0].classList[window.__THEME_MODE === 'dark' ? 'add' : 'remove']('darkmode');
}
if (!!window.matchMedia) {
 ['light', 'dark'].forEach(mode => {
  window.matchMedia(`(prefers-color-scheme: ${mode})`).addListener(e => {
   if(!!e.matches) {
    userTheme();
   }
  });  
 });
}
userTheme();