Another great online resource for people interested in building themes for the new editor is the live Q&A hosted by Birgit Pauli-Haack, last year. It can be reached at https://gutenbergtimes.com/live-q-a-block-based-themes-for-full-site-editing-feature-in-wordpress/.

By the way, the Gutenberg Times website, as well as the newsletter and videos, are very valuable for anyone interested in following the evolution of the block editor more closely. Highly recommended.