How to turn on Dark Mode on YouTube in the Android mobile app
YouTube’s Android app also follows the phone’s system-wide theme. But if you want to use only YouTube in the dark mode, you can change it in the app settings. Go to your YouTube app from your phone and open it.
When the application starts you will see your profile icon or a general icon if you are not logged in on to top-right corner of your phone screen. Tap on the icon and select the Settings option.
Click on General & Go to Appearance. From there select Light theme or Dark theme.