jrimko,
Themes are something we decided not to implement in core (the main library) because I think pulls it too far away from its core focus (i.e., making it monolithic).
For an integration package that handles this type of concern, you might want to look at hyla.
https://rubygems.org/gems/hyla Hyla works at the project level and thus can help manage resources such as themes/stylesheets.
I think hyla has the right approach of managing assets in a project. I don't like the idea of installing themes to the system because that makes builds non-portable. I think we've moved past that way of thinking as an industry. But, as I like to say, what I think doesn't matter. You can always create a tool on top that does exactly that. We encourage hacking.
Cheers,
-Dan