At the moment the dark theme is available on the YouTube website, soon perhaps it may be available in the apps too:

• Go to https://youtube.com/new (it can be an individual video or the homepage) and login if you have not already
  Click on your user avatar in the upper right corner of YouTube
  YouTube Dark Mode
  
  
• Choose “Dark Theme” from the dropdown menu
  YouTube Dark Theme
  
  
• Toggle the switch for “Activate Dark Theme” to the ON position, the interface change takes effect immediately
  YouTube Dark Mode theme enabled
  
  
• Now everything on YouTube will be dark, which makes watching videos in the dark or in the evening hours a bit nicer.
  

Dark Mode on YouTube enabled ...