In reply to Guido.

I actually see block theming being a return to classic theming's roots in many ways. This feels more like the "classic" that I remember from WP's earlier days than the complexity that has been added in recent years.

The big difference is really in the templating. Instead of HTML sprinkled with PHP tags, you have HTML with block comments.

As far as theme.json goes, you can opt into as much or as little of that as you want. This is particularly true of the styling aspect. You can go full-on configuring everything in theme.json or stick with style.css.