What else is there to theme? You can still go into Hamburger Menu > Customize and basically change every aspect of the browser.
Actully quite a lot. It seems moz took down the full themes, but I found this old site with some examples if you are curious http://www.smashingapps.com/2008/09/28/11-free-firefox-3-themes-you-probably-never-use-before.html there used to be a few hundred of these types of themes.
Over time moz slowly pushed for less customization, eventually people had to use 'classic theme restorer' addon to put this back into firefox in order to use the complete themes, but now even that is gone.