darkmode-3.js

function userTheme() {
 window.__THEME_MODE = 'light';
 if (!!window.matchMedia) {
  window.__THEME_MODE = window.matchMedia("(prefers-color-scheme: dark)").matches ? 'dark' : 'auto';
 }
  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();