Using ASP in CSS

I decided recently to tidy up my CSS for a web app that the company I work for is building. What I decided to do was gather up all the colours used and give them variable names so that if I, say, want to change the shade of grey that is "lightgrey" I can just do it once instead of hunting for all instances of #CCCCCC, for example.

I wasn't thinking last night and went and did this in PHP... the app is written in ASP. D'oh! What extension should I use and how do I link in a CSS file with ASP code in it?