Development on the Watson Design Guide is done in this repository. Patterns are developed in the patterns/ folder, with enhancements to the guides done in guides/ and enhancements to the site itself done in the site/ folder. The design guide runs off of the Watson Runner, so work on that should be done there.
Development is done through Node. Please refer to the Developing for the Design Guide wiki article for how to get started and how to work on the Design Guide.