Great thing is that as opposed to the web, the platform/library designers already took care of a lot of that, so the biggest overlap between GUI libraries and CSS is usually the layout (these days, where CSS isn't utterly abominable here).

A lot of the color fiddling etc. that web apps due is due to

1.) Graphic designers having no other way to go, now that print is dead

2.) Corporate Identity (which is of lower priority than platform identity)

3.) Selling stuff (not needed as much with desktop apps)