This addon/extension is not associated with YouTube. If you see anything other than YouTube on the page, then that is an error in the source code of the page you are viewing. This theme does not work on Safari browsers (it might work on Google Chrome, but not others).

This is an unofficial extension for use with the official YouTube website. It is intended to provide you with a more comfortable viewing experience and can be used without any further permissions. This extension adds a nice dark theme to YouTube with a smooth black background, which makes it much easier on the eyes especially during night time.