YouTube is planning to enable “dark mode” feature on mobile App for both iOS and Android. However, the company has already launched this mode on its website which gets most requesting feature om mobile app.
To enable this mode, you should tap on Account icon > settings and then you can select the dark theme from available option. This dar theme is going to launch today for iOS app according to The Verge. An Android version of this theme is coming soon. Post comments below if anyone sees this theme on iOS devices.