Enable YouTube's secret dark mode
UPDATE: As of 2-May-2017 this dark mode is now official in YouTube and you no longer need this extension! See here https://www.youtube.com/new
Enables YouTube's secret dark mode by setting a known cookie to enable server-side styling and a new page layout from YouTube itself. Unlike some other extensions, it does not change the HTML or style sheets after they are downloaded.
The first time it is enabled and you load youtube.com, it will display a desktop notification indicating the you'll need to reload the page, and enable Dark Mode in the settings menu to see it in dark mode.