The Url to the the .css file located within the theme folder for the them you use, will automatically be generated. So you don't need to worry about the Url to the css file. ASP.Net 2.0 will do it for you.. (or at least it should ;))
/Fredrik Norm?n NSQUARED2
Microsoft MVP, MCSD, MCAD, MCT
CornerstoneMy Blog, ASP.Net 2.0 etc