Remove 'stable' theme

Nice work @Liam Morland and @elber.

I suggest tagging a release asap.

I usually wait until issues auto-close before tagging a release candidate, but I'll leave that up to you if you want to do it earlier. Except for critical bug and security fixes, I wait until the release candidate is two weeks old and without release-blockers before making a full release.

Automatically closed - issue fixed for 2 weeks with no activity.