As long as you don't put anything in that extension that will steal private information, I think you will be allowed to do so.
I think I'd be too paranoid to use a bitcointalk extension for just that reason. I don't have enough technical knowledge to judge whether something like that is malicious or not, though I'm pretty sure I'd hear about it if it were. In any case, even aside from security issues I'm not really enthusiastic about any extension for the forum. Firefox already has a dark mode extension, which I use, and I kind of like the forum just the way it is--it doesn't need any enhancements IMO except maybe to make it mobile-friendly.
I just tried on chrome, if possible, I will do it for Firefox as well.
Although I myself am not interested, I'm sure many other members would be--so good luck on your project here. Maybe you'll have a hit on your hands.