Document what's available in theme.json vs in the Styles interface

[DaisyOlsen]

What is the new page you are requesting?

Copied from Related Gutenberg Issue

Right now, the process for determining which settings need to be done in theme.json vs the Styles interface is very manual. Tied to this, there's a regularly increasing amount of possibilities coming to each. For example, box shadow support was added to theme.json but it would be great to know when this can be controlled in the UI without needing to manually test or crawl through Gutenberg releases.

The goal here will be first to add the content to the Block Editor Handbook and then copy the portions that are present in core to the Theme Handbook. I think this will make it easier to keep things updated as changes happen.

I'm open to suggestions, and happy to take ownership of this issue, I don't have triage access at the moment, however.

How will this new page help you?

This page will help clarify what options are available in theme.json and indicate if those options are also available in the Styles editor. This type of reference would be helpful for anyone building themes or using the site editor to build sites and later extending via theme.json.

[zzap]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.

[github-actions]

Heads up @kafleg @carolinan @TeBenachi - the "themes" label was applied to this issue.