Browser Script: Youtube Cinema Mode. Adds an invisible checkbox to the top-right corner of youtube.com that hides everything but the video. You can add this code to extstyler.js in Extstyler to use this script in Chrome https://github.com/tomhodgins/extstyler

youtube-cinema-mode.js

// Run code on Youtube.com
/* Youtube Cinema Mode */
if (document.getElementsByTagName('html')[0].classList.contains('youtube')) {
  var input = document.createElement('input'),
      inputCSS = document.createElement('style'),
      inputJS = document.createElement('script');

  input.id = 'ytCM';
  input.setAttribute('type','checkbox');
  input.setAttribute('onclick','cinemaMode()');
  if (localStorage.ytCM == 'true') {
    input.setAttribute('checked', 'yes');

  }
  inputCSS.innerHTML = '#ytCM{position:fixed;top:0;right:0;z-index:20000000000;opacity:0;transition: all .1s ease-in-out}\
                        #ytCM:hover{opacity:1}\
                        #ytCM:checked ~ #body-container #yt-masthead-container,#ytCM:checked ~ #body-container #masthead-positioner-height-offset,#a11y-announcements-container,#ytCM:checked ~ #body-container #watch7-container,#ytCM:checked ~ #footer-container {display:none}\
                        #ytCM:checked ~ #body-container {background: black}';
  inputJS.innerHTML = 'function cinemaMode() {\
                        if (localStorage.ytCM == "true") {localStorage.ytCM = "false"}\
                        else {localStorage.ytCM = "true"}\
                      }';

  document.getElementsByTagName('body')[0].insertBefore(input, document.getElementById('body-container'));
  document.getElementsByTagName('body')[0].insertBefore(inputCSS, document.getElementById('body-container'));
  document.getElementsByTagName('body')[0].insertBefore(inputJS, document.getElementById('body-container'));
}