Dark Reader extension works pretty well for FireFox (I'd recommend switching away from Chrome after they pulled the whole Manifest V3 thing anyway).
How to change the dark mode
Read my previous comment. You can’t without a browser extension.
Seems like an easy thing to implement.
GWM will is a side project, not a primary job. Also, there is a difference between “easy” and getting it right. Getting it wrong would mean more people complaining about the issues. There are many features that may be “easy” to implement, but the devil is the details and it isn’t a priority right now.
It should be noted, on of the mods had done an experiment, and there are plenty of edge cases that actually make it not so straight forward.
Is there a way for users to change the background color or "theme" of the site display? I would very much like the background color to be a dark color, but I don't see a way to do that in the Account Settings.