I have used this idea with other web design projects and it worked great: take all of the color declarations out of a css file and put them into another css file called colors.css. Then call both the styles and colors file into the template you want.
 
Why? Because, then you could have one template, one styles sheet (giving general styles), and multiple color files that could customize each site but still have a unified feel between all your sites.
 
Many big online retailers do this.
 
Bill

I use multi sites but even without that I would like to see this - and as a step to making css files more usable in general.