Jeff Robbins posts about dynamic CSS. The idea being creating a CSS in php where you can use variables such as changing colors, backgrounds etc. in the PHP. It also has potential for the ability to reduce the number of files necessary to serve a page.
In systems like Drupal where there is a system-wide style sheet as well as one created for each theme and some modules add their own as well. With all of these it means more connections required to download a single page. For many users this is not a huge deal. For those of us on satellite links it is a problem.