@2fd7551e I don’t have an answer to this. 

But I think the ultimate point of the FSE is that there no themes in the traditional sense. Just a customized theme.json. 

I don’t think we’re quite there in 6.3. But soon. 

 @24ce6141 that's not how I understand it based on this page - https://developer.wordpress.org/block-editor/how-to-guides/themes/#types-of-themes

Seems to indicate that it needs a template, not a theme.json