dark.js

document.addEventListener('DOMContentLoaded', function(event) {
  var changeColorScheme = function() {
    var html = document.querySelector('html');
 
    if (this.matches)
      html.classList.add('dark');
    else
      html.classList.remove('dark');
  }
 
  var mm = window.matchMedia('(prefers-color-scheme: dark)');
  mm.addEventListener('change', changeColorScheme);
  
  changeColorScheme.call(mm);
});