Ok, the forum is getting a facelift.
The old themes are still available for the moment, but the site default has been changed.
If you want to keep using one of the old themes, you can still select that permanently in your profile options.
The theme selector gizmo at the bottom of the left column is session-dependent, not permanent.
Post any grumbles if you find some thing broken or unusable. Questions are fine too.
Any feedback is welcome, but please try to make it specific enough to be useful.
The new theme has four different backgrounds available, in a selector thingy you can find just under the FaceBook/Twitter/YouTube/RSS icons at top right.
There are also four layout options. Mobile and smallscreen are still a bit WIP, but there's nothing like making something public to get more bug reports.