YouTube is now rolling out its much anticipated Dark theme to its Android app. The theme is designed to reduce strain on the eyes especially under low light. The feature is already available in iOS and desktops. The new Dark theme is available in the latest latest YouTube app (version 13.35.51) on Google Play, though availability varies from region to region.
The theme can be enabled to change the background of the the home screen and search pages to dark grey in colour. The font also changes changes from black to white. The feature helps to reduce strain on the eyes under low light and save some battery life as it does not use white pixels.
After you have updated the app for your Android phone, you will get a pop up notification introducing you to this new feature. Then you can tap on the top right corner of your app and go to Settings > General. The new feature is rolling out in phases and may not be available to some users even after downloading the latest YouTube app.