YouTube - Always Theater Mode

Set the default viewing mode to Theater Mode.

As of 2015-06-27. See the latest version.

// ==UserScript==
// @name        YouTube - Always Theater Mode
// @namespace   r-a-y/youtube/theater
// @description Set the default viewing mode to Theater Mode.
// @include     http://www.youtube.com/watch*
// @include     https://www.youtube.com/watch*
// @version     1.2
// @grant       none
// ==/UserScript==

document.getElementById('page').className = "watch watch-stage-mode";
document.getElementById('player').className = "content-alignment watch-medium";
document.getElementById('watch7-container').className = "watch-wide";

var playlist = document.getElementById('watch-appbar-playlist');
if (playlist) {
  playlist.setAttribute('style', 'top:520px');
}

// html5
var video = document.getElementsByTagName("VIDEO")[0];
if (video) {
  video.setAttribute('style', 'width:100%; height:100%;');
  document.getElementsByClassName('html5-video-content')[0].setAttribute('style', 'width:100% !important; height:100% !important;');
  document.getElementsByClassName('html5-video-container')[0].setAttribute('style', 'position:static');

  // props Kooilnc from Stackoverflow
  // http://stackoverflow.com/a/2706236
  function eventFire(el, etype){
    if (el.fireEvent) {
      (el.fireEvent('on' + etype));
    } else {
      var evObj = document.createEvent('Events');
      evObj.initEvent(etype, true, false);
      el.dispatchEvent(evObj);
    }
  }

  // emulate clicking on the "Theater Mode" button for HTML5 progressbar to expand
  eventFire( document.querySelector('.ytp-size-toggle-large'), 'click' );
}