Greasy Fork is available in English.
Select a youtube resolution and resize the player.
It doesn't default to theater mode for me. Is is possible that you are part of a youtube experiment, or have another addon that is doing it? If it is the case that people are getting theater mode by default now, I suppose I wouldn't be opposed to adding an auto-off feature even though I think a lot less people would be interested in it. But if I'm not getting it myself then I can't really test it so I would like to be able to have this happen to myself before I add anything.
Interesting. I get this in the page load
<script nonce="redacted">(function() {window.ytplayer={};
ytcfg.set({"CLIENT_CANARY_STATE":"none", [...] "START_IN_THEATER_MODE":true, [...]
There is no way to easily override it since it's an inline script. It seems to be tied to some setting. It's false when logged out. I wonder where you could set it. ytcfg.data_.EXPERIMENT_FLAGS is massive but nothing looks like it's related.
Cool script. Now that theater is the default could you make the opposite? So that it turns theater off.