I'm not a web developer so please correct me if I'm wrong. This sounds like a limitation of the tmLanguage file (which applies syntax coloring). If that's the case, it's something that a user can create (and ideally share). If you are already using a third party tmLanguage file, you should make sure that it is actually being used. To do this, you can add the "CSS" package to the ignored list. Alternatively, you can try using the ApplySyntax
plugin. I'd be surprised if no one had created an updated CSS syntax highlighting file (since it is so popular). That being said, it's still possible.