In reply to Cathy Tibbles.

I meant to link to an earlier post, Initial Documentation for Block-Based Themes Proposed, that explains how they will work. Of course, so much of this could change in the coming months. I just went back and added the link into the post though for reference.

Some other posts that might be worth digging into that touch on the subject:

Key Takeaways from the Future of Themes Meeting
Get Involved with Block-Based Theme Experiments