The three content interfaces on gtk-common-themes should all be ready for the greedy connection assertions.
As far as autoconnection for third party theme snaps, I am a little concerned about granting that until review-tools gains theme snap specific checks, as I outlined here:
If we are going to automatically prompt users to install these snaps to handle their selected theme(s), then I want to be reasonably sure there are no other side effects. As well as checking that it shares the content we expect it to, we also want to be sure it isn’t bundling anything unexpected like daemons or commands.