This was very helpful in Photoshop plugin development: https://developer.adobe.com/photoshop/uxp/2022/guides/theme-awareness/
It would be very useful on its own for InDesign plugin development. And, of course, it would also simplify life for those of us that would like to have a plugin targeting both applications with a single codebase.
These do not appear to be working currently, either in 18.5 or in the prerelease 19.0 I am currently using for testing. e.g.
--uxp-host-text-color is showing black in all themes.