If you want conditional CSS to be inside the external CSS file (which is really the best place for it) try
From here you can download a PHP class that builds your CSS file, allowing you to use simple conditional statements for blocks or single lines.
This is definitely the time saver!
PHP conditional CSS is somewhat ‘old-hat’ now that CSS3 and Modernizer are doing their stuff. Also check out LESS and SASS for building your CSS.