A lot of that stuff is built into the browser, for people who care to look through the customization options and possibly do a tiny bit of about:config tweaking.
Re:Firefox 57.0 Released
by electrotype on Tuesday November 14, 2017 @10:23AM
(#15695759)
Have a look at https://github.com/Aris-t2/CustomCSSforFx , it can do many things, even if it's not a perfect Classic Theme Restorer replacement.
https://bugzilla.mozilla.org/show_bug.cgi?id=1328244