Racket is basically like Scheme when it comes to syntax highlighting, it only has some minor differences. I have made a complete Racket grammar (link), but I think just copying the Scheme one and adding support for racket language will be cleaner and function just as well.

If you want me I can open a PR, just need to decide if we want the complex grammar or rather the simple one.

EDIT: Updated the link to point to the renamed repo.