As far as just colors and font size and such, that used to be an OS feature that every (native) application got for free. FF had it years and years ago, too. I

As far as just colors and font size and such, that used to be an OS feature that every (native) application got for free. FF had it years and years ago, too. I tried it a bit back then and then stopped, because almost all the themes just made it harder to use. Not sure what's up with their making such a big deal out of adding a weaker and less-customizable version of it recently with their color themes. "We redid an old feature, but worse".