theme chooser for cm 12 broken/disappeared after update?
After using adb to remove the data for the packages org.cyanogenmod.theme.chooser and org.cyanogenmod.themes.provider, as suggested above and elsewhere, and also after cleaning the data in trebuchet on the phone (by way of trial and error), the only thing that really made the Theme Chooser work again was a clean flash:


download the latest cyanogenmod (and Google Apps if you want) to your sd card (remove /system/* too if you are switching to Open GApps from another GApps)
reboot into recovery (on an Xperia Z1 Compact by repeatedly pressing volume-up at boot)
choose wipe data / factory reset and empty cache
do install update for both the CM and GApps ZIP files
reboot


After reboot, my apps were re-installed via my Google account and the data (images, documents) from before flashing were all still there.

The custom wallpapers and (in my case no) sounds needed to be set anew.