It really bothers me that I cant find the command for changing sddm theme.
(maybe it was some qdbus thing …)
Anyhoo … Its easy enough to go into /etc/sddm.conf.d/kde_settings.conf and edit for default breeze theme under the [Theme] tab:
Current=breeze
(the above path assumes plasma settings created the config … if not … it may simply be under /etc/sddm.conf)