Gutenberg 9.1 made a backward-incompatible change to its theme.json file (experimental-theme.json while full-site editing is under the experimental flag). This is the configuration file that theme developers will need to create as part of their block-based themes. Staying up to date with such changes can be a challenge for theme authors, but Ari Stathopoulos, a Themes Team representative, wrote a full guide for developers.
Jon Quach, a Principal Designer at Automattic, has also been busy creating a tool to help theme authors transition to block-based themes. He recently built a UI-based project called Theme.json Creator that builds out the