Greasy Fork is available in English.
AKA: "Wide Youtube", AKA: "Wide video container" - Uses the page space on youtube more efficiently (especially good for high resolutions)
Feature added. Only for theater mode for now though.
I hope that works for you. I personally don't use anything but the theater mode.
Might see about fixing up non-theater mode later.
This would be even better if it worked on the page when you are already watching a video to make better use of the space on the sides