User-Script: YouTube Dark & Wide Mode.

dark-youtube.js

// ==UserScript==
// @name         Dark Wide YouTube
// @namespace    DWYT
// @version      1.0
// @description  Dark Wide YouTube
// @author       Syed I.R.
// @match        https://*.youtube.com/*
// @require      https://raw.githubusercontent.com/madmurphy/cookies.js/master/cookies_min.js
// @grant        none
// @noframes
// ==/UserScript==

(function() {
    'use strict';
    const FUTURE = new Date('2020');

    // Dark Mode On
    let pref = docCookies.getItem('PREF');
    if(pref) {
        let cookieChanged = false;
        if(pref.indexOf('f6') === -1) {
            pref = pref + '&f6=400';
            cookieChanged = true;
        } else if(pref.indexOf('f6=400') === -1) {
            pref = pref.replace(/(f6=)[^\&]+/, '$1400');
            cookieChanged = true;
        }

        if(cookieChanged) {
            docCookies.removeItem('PREF');

            docCookies.setItem('PREF', pref, FUTURE, '/', '.youtube.com');
        }
    }

    // Wide Angle
    let wide = docCookies.getItem('wide');
    if(!wide) {
        docCookies.setItem('wide', 1, FUTURE, '/', '.youtube.com');
    }
})();