I found this bug in the latest Linux Mint Xfce edition -- the Arc-Colors themes, which are MDM (Mint Display Manager) themes, are showing up in the theme list in the appearance settings, and they have no business being there. Deleting those themes solves the problem -- but that also makes them unavailable to choose as an MDM theme.
I don't know what MDM expects, but if it looks for themes in the same locations as our appearance settings then there is little we can do.