Change Appearance

I have searched high and low for an answer to this simple question and tried to implement every (conflicting!) suggestion offered by Google AI and the Firefox help facility without success. None of the suggested fixes lines up with what I'm actually seeing on the screen at all. All I want to do is change the "theme" or appearance of Firefox so that it follows the default OS settings -- in this case, I want green highlights and accents instead of Firefox's chosen purple. That's it. It really shouldn't be this hard, and it's incredibly frustrating to have to go down so deep a rabbit hole for what should be such a simple and obvious setting. If someone can take the time to explain like I'm five the steps I need to take, I'd be most grateful.