I think the UX of requiring users to opt out of untested and likely broken themes on a per app basis is bad. I think an opt-in via prompt on first run (or what

I think the UX of requiring users to opt out of untested and likely broken themes on a per app basis is bad. I think an opt-in via prompt on first run (or whatever) is a better experience since it makes the distinction clear in case stuff breaks and provides an opportunity to warn users about potential breakage.