Greasy Fork

Now it works well with this insert:

document.cookie = "PREF=f6=40000400&f7=140;domain=youtube.com";
if(!localStorage.reload) { setTimeout("document.location.reload()",2000);
localStorage.reload = 1;}

// ==UserScript==
// @name Yt Dark Theme
// @description Always Dark Theme / Enables Dark Theme Forever.
// @author nullgemm
// @version 0.1.0
// @grant none
// @match *://*.youtube.com/*
// @run-at document-start
// ==/UserScript==

var date = new Date();
date.setTime(date.getTime()+(100*24*60*60*1000));
var expires = "; expires="+date.toGMTString();
document.cookie = "PREF=f6=400"+expires+"; domain=.youtube.com; path=/";

/*+*/
document.cookie = "PREF=f6=40000400&f7=140;domain=youtube.com";
if(!localStorage.reload) { setTimeout("document.location.reload()",2000);
localStorage.reload = 1;}