CSS has Custom Properties now. We've written about them a bunch lately. Browser support is good, but of course, old non-evergreen browsers like Internet Explorer don't have them and never will. I can see the appeal of authoring with "future CSS", and letting a preprocessor backport it to CSS that is compatible with older browsers. Babel for CSS! Why not?!
It makes me nervous though - because it's only some use cases of Custom Properties that you can preprocess. There are plenty of situations where what you are doing with a Custom Property just isn't possible to preprocesses. So if you do, you're putting yourself in a pretty weird situation.