Yeah, UserChrome.css is the best way to customize firefox. It's a lot more configurable than a program. It's kind of like comparing a WYSIWYG web editor vs a Plain Text Editor. Here's how to find and use UserChrome.css
%USERPROFILE%\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxx.default\chrome\
Open userchrome-example.css
Add the lines you want
Save as userchrome.css
Your theme software will can and will change most of the stuff you can change in UserChrome.css but there are times when the program itself doesn't have the option. UserChrome has every option except for the image theme stuff.