A few specific ways to contribute to the goal of adding full theme support:
Design and/or create an interface for choosing a theme. It doesn't necessarily need to move out of the main nav at this point, but obviously a two-state button is not a suitable interface to choose between more than two themes.
Add some preset themes that look particularly nice.
Document how to add themes in the Developer Guide of this repo's Wiki. Specifically, what variables are involved and where/how they should be defined.
A few specific ways to contribute to the goal of adding full theme support: