Comment 17 for bug 1034755

I tried to report this to plank's page and the guy marked it as a duplicate of THIS bug even though elementary isn't involved in the project. In any case, nothing is broken. The themes themselves are broken. In /usr/share/plank/themes, check the themes you have added. Their .theme files use [titles] that end in "renderer," but the current plank no longer recognizes them so it always reverts back to the default.

Delete "renderer" from all [titles] and then manually change the theme. That should work.