It looks like Google is testing Dark Mode for all its apps like the Dialer, Contacts, Messages, etc. Finally, the official YouTube app for Android is getting the much awaited Dark Mode, and this new update is now rolling out for everyone. So, it is no longer available to only a few users like before. Just like the new Android Messages app, the Dark Mode applies a Dark Grey theme to the overall User Interface. Some people may still prefer using the YouTube Vanced application since it lets you have a Full Black theme that looks much better.
As you can see in the screenshot above, you’ll need to head over to the YouTube App’s Settings > General. There, you’ll see a new option that says ‘Dark Theme.’ Toggling this ON will enable the dark theme throughout the YouTube app. Now, for some reasons, you may not get this option. According to some users, clearing the cache and the data of the YouTube app fixes this issue, and they were able to see this option in the settings.
Again, if this method didn’t work for you, make sure that you have the version 13.35.5 or above of the YouTube App on your phone. You can update it directly from the Google Play Store or get the APK file from the APKMirror.