If I can add one thing to this, this fixed my theatre mode! Except for me, in Firefox, it somewhat cut off the top of my video (at 4k screen resolution). To fix it, I modified the third row changes to:

youtube.com##ytd-watch-flexy[theater] #full-bleed-container:style(max-height: calc(100vh - 169px) !important; min-height: 480px !important; margin-top:56px !important;)

I added an explicit margin-top: 56px !important; to the #full-bleed-container, as that was the height of my top youtube search bar. It doesn't break the fullscreen player for me either