This extension forces YouTube to open its player in the theater mode by default even if you are not logged-in. You can simply switch back to the normal mode by pressing the button in the player, however, next time you open youTube it will be in the theater mode again! The extension does not have any options. It just works. Note that I am not using CSS or JS tricks to simulate theater mode. This is actual theater mode that YouTube offers.
To have this option in the incognito mode, you need to allow extension to be active on this mode as well.
For bug reports, use the discussion section of
http://add0n.com/youtube-tools.html?theater