This can be done server-side and save you a bit of hassle if that's an option. Since you're just merging the files together and since it's just CSS there shouldn't be any conflicts in the information that should harm your site. Also, this way gives you flexibility to make updates to the CSS as frameworks are improved.
Ruby is not my language of choice but still very viable to do everything needed here. There is a tool out there written in Ruby that will do this for you with CSS as well as JS files. Take a look at this blog post to get the rundown:
http://cjohansen.no/en/ruby/juicer_a_css_and_javascript_packaging_tool
I hope that this is helpful, and please let me know if you need anything else on this one.
