Go the youtube homepage hit Ctrl+Shift+J for google and Ctrl+Shift+K for firefox users a developer console should pop up if it has code and a bunch of nonsense already there right click then choose clear console now copy this code and drag it into the console
document.cookie="VISITOR_INFO1_LIVE=qDpUsBNO0FY; expires=Thu, 2 Aug 2020 20:47:11 UTC";
Hit enter on the keyboard and and refresh your page it should revert back to the original theme!