Thank you for being interested in contributing!
First, see if an issue has been reported. Create a new issue if no one has reported on the same topic. Follow the issue reporting template.
Fork and clone this repository into your local development environment. For color-specific tweaks, you'll want to apply the necessary changes in the themes folder.
Manually test your changes by launching the extension (Fn + F5 by default). Once you're done testing, create a pull request and link it to the issue you created (or discovered) earlier. Abide by the pull request template.