The YouTube app for iOS will get the night mode in one of the following updates

The developers of YouTube will add a night theme to the official application for iOS devices in one of the next updates. This informed informed user Reddit Dinn Cobb. 
He posted a screenshot of the application settings version 13.01.4 (currently available in the App Store 13.01), which in the options list has the item "Night mode". It is reported that an update with this feature will begin to arrive in the near future to a limited number of users. When a similar feature begins to be tested for the Android version of the application - is unknown.
At the moment, the night theme is available to all users of the service only in the web version of YouTube. To enable it, you need to click on the avatar in the upper right corner and activate the function "Night mode".