This style also hides the top nav bar to give the video more room (and to make the implementation easier for me). If you need to use the nav bar, go to a non-video page, or disable the style temporarily.
I recommend using https://chrome.google.com/webstore/detail/auto-hd-for-youtube/koiaokdomkpjdgniimnkhgbilbjgpeak/reviews along with this style, so that you can automatically use a higher resolution for the video to take advantage of the larger display area this style provides.

known issues:

• the playlist gets pushed down the page some
  
• when the window is narrow enough for the horizontal scrollbar to appear, it will appear on top of the video
  
• the progress bar and annotations will not scale along with the video. there doesn't seem to be a way to fix this with CSS, as explained in https://github.com/Zren/ResizeYoutubePlayerToWindowSize/issues/6
  

please report any other issues https://github.com/X1011/youtube-theater-mode-4realz/issues

changelog:
2015-05-19: fix for moar YouTube changes!
2015-05-05: fix for YouTube changes, fix for scrollbars at some smaller widths
2015-04-21: fix for changes YouTube made to the design at small window sizes
2015-02-07: fix "video unavailable" pages (4realz this time)
2014-11-12: fix centering of non-16:9 videos
2014-10-16: fix "video unavailable" pages
2014-10-04: fix for http://freestyler.ws/style/55584/black-youtube-by-panos