Update: Installing gnome-settings-daemon got xubuntu-gdm-theme to work again. Removing (not purging) gnome-settings-daemon caused the xubuntu theme to disappear again. With this in mind, shouldn't gnome-settings-daemon be a proper dependency instead of merely a recommended package? Or, perhaps xubuntu-gdm-theme can be made to work without it by setting something up in a postinst script?
Update: Installing gnome-settings- daemon got xubuntu-gdm-theme to work again. Removing (not purging) gnome-settings- daemon caused the xubuntu theme to disappear again. With this in mind, shouldn't gnome-settings- daemon be a proper dependency instead of merely a recommended package? Or, perhaps xubuntu-gdm-theme can be made to work without it by setting something up in a postinst script?