Re: [development] Move multiple theme per site support to contrib module

but having the notion of "enabled" and "default" theme is essential to us, we use groups on our sites and having the ability for each group to choose their own theme seems a kicker, people feel that's a space belong to themselves. If there are other ways to achieve the same result it will be fine, but we don't want to lose the user specified theme capabilities -- i.e. make sure the contribute module for multi-theme support is available at the same time when it disappears from the core.

if you are referring to organic groups module, then it will surely continue to offer theme choice among the non enabled themes. no worries.