SemanticUI -- how to compile new theme?

I ran through the instructions here:
http://learnsemantic.com/developing/customizing.html
and here:
http://learnsemantic.com/themes/overview.html

However, I see that there are quite a few .less files in the ~\ResourcePackages\SemanticUI\assets\src

How does one build these into a SemanticUI theme? When I compile my theme, I'm missing the Sitefinity overrides in these less files.
