YouTube - Always Theater Mode

Set the default viewing mode to Theater Mode.

As of 2017-09-11. 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     https://www.youtube.com/*
// @version     1.4.0
// @grant       none
// @run-at      document-start
// @license     GPL v3
// ==/UserScript==

// Toggle Theater Mode after YouTube finishes.
window.addEventListener("yt-navigate-finish", function(event) {
	var newPlayer = document.querySelector('button.ytp-size-button');
	if ( newPlayer && null === document.querySelector('ytd-watch').getAttribute('theater') ) {
		document.querySelector('button.ytp-size-button').click();
	}
});