Here's how to contribute.
Extract and copy files to your ~/.config/geany/filedefs directory. Versions of geany older than 0.16 use the ~/.geany/filedefs folder instead.
Instructions for creating and installing Geany schemes can be found here: http://www.geany.org/manual/current/index.html#filetype-definition-files
The simplest way to create a filedef for a new language is to copy an existing one from the default install and change the colours.
Fork the repo and submit pull requests!
You can discuss geany-dark at the geany Google Group: http://groups.google.com/group/geany