There's a quick, handy trick over on PHPComplete about making your CSS download faster as optimized files.

Actually, pretty much anything can be optimized with PHP, at least for download times. Take CSS files. I was able to take a 2000+ byte CSS file to just 681 bytes. I used the classic ob_start() method by placing this at the beginning of the CSS file [...] And then, in an .htaccess file, I put: AddType application/x-httpd-php .css

That is placed in the CSS directory, and it will run CSS files through PHP

Simple, handy, and easy - what more could you ask for...