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Set the default viewing mode to Theater Mode.
От
// ==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
// @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";
document.getElementById('watch-appbar-playlist').setAttribute('style', 'top:520px');