Hi,

Scheme isn't in the currently supported languages, I don't know if anyone is doing a scheme/lisp for Geany.

To create one yourself see hacking on the Geany website which outlines the process.  As explained there you will need a scheme/lisp lexer for Scintilla, the editing component used by Geany. You may be able to use or base one on the lexers from the Scite editor which uses Scintilla and supports lisp&scheme.  Then you can create the filetype.scheme file which specifies the styling you want to apply to the entities that the lexer parses from your source file.

Cheers
Lex