bookmarklet-toggle-external-css.js

(function(){
  var stylesheets = document.querySelectorAll('link[rel=stylesheet][href]');

  Array.prototype.forEach.call(stylesheets, function (element) {
    if (element.media === 'x') {
      element.media = element.getAttribute('data-media');
    } else {
      element.setAttribute('data-media', element.media);
      element.media = 'x';
    }
  });
}());