FSE: support for child themes

[oandregal]

The next default theme TwentyTwentyOne (TT1) will explore FSE editing support, including Global Styles. It's also thought of as a teaching tool for other people to create child themes.

Ideally, child themes would also be able to override parts of its parent theme.json without having to write a new one.

[aristath]

Just a note here: It's not just parent-themes and child-themes... It's also possible to have grandchild-themes, so this will probably need to be recursive and go up the themes-inheritance tree.

[ZebulanStanphill]

@aristath While I can't find any definitive source, I thought WordPress never officially supported "grandchild" themes.

[aristath]

Officially yes... But unofficially they do work fine provided the grand-child has minor changes and they are not that rare. With FSE themes it will be even easier (and in some cases even desirable) to have grandchild themes

[oandregal]

Going to pause #27305 for now. I've seen there's rapid movement in the FSE/templates area and I believe when that's more settle I can resume this work.