The app version is in use by quite a few people who don't have dark mode which leads us to believe it's a server side switch - so expect the long slow trollout we've come to expect from Google. For reference, on iOS devices you can find it under Settings Dark theme.
Given that both iOS and Android device owners like to bring their phones to bed with them, it's nice to see Google extend this feature to the rest of their user base. There are no additional changes in terms of design when you apply the new theme, except for the obvious darker color, which isn't really black but more of a dark gray.
If you own an Android device and are not yet seeing the option, you can check if it is available by going into Settings General.
YouTube's new look is much cleaner and arguably easier to navigate, but there's more to the redesign than meets the eye. The same goes for the minimized video player and all the tabs.
When the feature has fully rolled out, the option can be turned on and off by the user. The search giant is apparently making good on that promise as dark mode has appeared on some handsets running Google's mobile operating system. If the feature is available, you will be notified about it when you open the YouTube app. Would you rather stick to the default white theme?