The first thing that comes to my mind when I see things like this is: is CSS really that hard? I don't say that in a condescending way, but does it really need to run through a PHP (or C) processor, adding unnecessary overhead? I think it is good for some systems to output the CSS based on variables in their backend CMS or storage system, but it should be cached as to not require parsing with each request. In this instance, the CSS doesn't change based on variables coming from a backend system, it only changes based on the browser requesting it.
I can slightly understand the need for @variables inside of CSS, but I just don't get the point of adding a middleman parser to do things like this. Am I interpreting it wrong?