YouTube on mobile to integrate with Android 10's dark theme soon
YouTube already has a dark mode that you can toggle within the mobile app. But soon it'll integrate with the system-wide dark theme Android 10 is offering. Some users see the change in their devices already, but it hasn't come to all users yet. YouTube is rolling it out the update on the server-side, so you can't force the update with an APK.
The change renames the "Dark theme" option in the settings to "Appearances." Within that option, you now get to choose "Use device theme," "Light theme," and "Dark theme." If you opt for the device theme option, YouTube's look will match what system-wide option you've set for your device.
Source: Android Police