The recent update of the Racket lexer improved the highlighting of Racket source code (rkt, scrbl, etc).

Consider reusing the Racket lexer to get better hightligthing of Scheme source (ss, sls, scm).

Consider this file:
https://github.com/akeep/scheme-to-c/blob/master/c.ss

Here square brackets are hightlighted as errors.