I am trying to focus on a single CSS framework for FolkBot and some implementations in distributions, binaries, products and SAAS.
There is a very excellent, non free, paid and premium theme, called Material Design for Bootstrap. It actually has a fremium kind of model. It has a well structured license and business model, BUT, none of the options allow embedding in an open source project, and since I want complete freedom I have chosen another implementation.
This Material Design for Bootstrap 4 by an Australian web agency called Daemonite. This seems the most active, if not the most popular, and since Bootstrap 4 is still in development I want a responsive team behind the codebase. It seems to have all the components and kitchen sink with great docs.