Givin theme.conf in /usr/share/sddm/themes/breeze-openSUSE with 

[General]
type=image
color=#1d99f3
background=components/artwork/1920x1080.jpg
needsFullUserModel=false

the background (green "artwork" on the right-hand side of the sddm screen) is
not loaded and/or displayed at all. Trying to load the file via kcm_sddm does
not help. It seems that this is caused by Main.qml in
/usr/share/sddm/themes/breeze-openSUSE. In fact, replacing this file, for the
sake of curiosity, by the corresponding one from Leap 15.0 solves the issue.