Ability to change themes based upon a layout

Description of the need

I recently ran into a use case for forcing the site to use the admin theme for all "Book" pages. I thought this would be easier than it actually is. My initial impulse was to make this conditional on content type, that is that one could specificy which theme to use for a specific content type.

After discussing this with others, we are thinking that it might be better to make this option available on a per layout basis. We could add an option to layouts that allows a person to select a different enabled theme for any specific layout. This would allow the site admin to create a Content Type specific layout that uses another theme, but they would also have access to all the other visibility conditions, including ROLE or specific individual paths.

See this forum discussion: https://forum.backdropcms.org/forum/using-admin-theme-book-pages

Proposed solution

Add a checkbox to the configure page for individual layouts: [ ] Use a custom theme for this layout

If this checkbox is checked, one could then select from a drop down list of enabled themes to choose an alternative theme for that layout.

Alternatives that have been considered

This could be a contrib module, but I think this MIGHT also be reasonable new feature in core. Existing contrib modules that do something related are:

Page Theme https://github.com/backdrop-contrib/page_theme

Content Theme https://github.com/backdrop-contrib/content_theme

Additional information

https://forum.backdropcms.org/forum/using-admin-theme-book-pages

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability to use different themes for specific layouts. Site admins will be able to choose any enabled theme to be used when viewing pages with a specific layout.