website-dark-light-switch.js

(function(){
  var styleId = 'invert-style';
  var style = document.getElementById(styleId);
  if (style) {
    style.parentElement.removeChild(style);
  } else {
    style = document.createElement('style');
    style.id = styleId;
    style.textContent = '' +
      'html,img, video, object, [style*=url] {' +
      '-webkit-filter:invert(100%) !important;' +
      'filter:invert(100%) !important;' +
      '}' +
      '' +
      'body {' +
      'background:black;' +
      '}';
    document.body.appendChild(style);
  }
}());