Isn’t that pretty much what content interfaces already do today? Their point is precisely to mount content across snaps, in a pretty similar way to what was described there.
The in-progress layouts feature will allow arbitrary remappings, but we don’t even have to wait for that. The content interface can already land the theme in a place the application can look for.
For details on which names to pick, the guidelines discussed around the gnome platform content interface seem to apply here as well, as there’s an implicit API that needs to be agreed on for the theme to work. I suggest reading the topic from the bottom up.
Then, we need to agree on some sort of convention so that something in the system, potentially snapd itself, can find out which theme would best match what’s available in the system, and then pull the snap in together with something that might make good use of it.
I won’t be around in the next couple of days, but would be happy to have a call or a more extensive conversation here next week, so we can evolve those ideas.