As it turns out, there is a way to turn on dark mode, as discovered by Owen Williams. Williams shared his excitement about a hidden dark mode on his Twitter account, sharing a browser screenshot that shows YouTube with a fully functional dark mode running.
To enable dark mode, open the console while you’re visiting YouTube and paste the following: document.cookie=”VISITOR_INFO1_LIVE=fPQ4jCL6EiE”. Once that’s done, all you’ll need to do is reload the page, click your avatar, and dark mode should load up. It’s a fairly straightforward process, though not quite as simple as flipping a switch.
YouTube URL will load the page with a dark UI, achieving a similar effect without having to change things in the console. While this is just a UI that YouTube has created for YouTube Gaming, adding “gaming” as a subdomain to the URL works on any of YouTube’s videos.
So there you have it: a few ways to get a dark UI on YouTube