[schrute]

That seems like it should be at least a customizable setting and IMO on by default. Let’s hope the HN thread full of people pointing it out helps.

[foepys]

Themes have control over how it looks. You can search, test, and install one directly within Firefox.

[mmis1000]

Not really, theme is currently only allowed to alter css variables(basically colors). There is a special type addon that do allow to alter the browser theme directly (behave like userChrome.css). But that requires addon signing from mozilla and enable certain hidden control in about:config. Basically not likely done by a general user.