It was a couple of months ago that we reported on a 'dark mode' for YouTube rolling out to Android users. Back then, it was only available to a select group of folks, but now, the option looks to be finally rolling out to all.
If you aren't familiar with dark mode, what it will do when enabled is take YouTube's traditional menu theme that is white and darken it to make it easier on the eyes when watching in low light or a dark environment. If you haven't received the prompt yet to try it out, you can try to enable the setting yourself by heading into the Settings menu.
To do this, open the YouTube app, head to the 'Settings' menu, then go to 'general', and switch the toggle to enable 'Dark theme'. After you do this, you should see the theme of the app change to a much darker color scheme. If you don't see the option to change it, you might have to perform an update, downloading version 13.35.51, which can be found by heading to the Google Play Store.
Source: Android Police