Probably a better idea would be to decide the appropriate content based on various criteria on the server-side. Otherwise the stylesheet will become a nightmare to maintain and would contain a lot of inappropriate logic which doesn't belong to css.
It can be used during development, as a runtime or buildtime solution. Also it can be used as a command line tool. It supports almost all known processors (less, sass, coffee, etc) and allows static code analysis with jshint, jslint or csslint.