Personally, I like the dull, drab grey scrollbars but, for those of you who don't, here ya go.
this is a GREAT extension. BTW, if only background colour works for you (arrow buttons and the bar does not work) then you should make small changes to the style code (add "-moz-appearance: none !important;" rule):
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
scrollbar * { -moz-appearance: none !important; background-color: red ! important; }
scrollbar thumb { -moz-appearance: none !important; background-color: blue ! important; }
scrollbar scrollbarbutton { -moz-appearance: none !important; background-color: yellow ! important; }
Simple: Firefox is controlling the scrollbars to look native, thus rendering the style useless.
Only the background colour works. The arrow buttons and the bar are still grey.