Comment # 6 on bug 1088284 from Wolfgang Bauer

(In reply to Frank Kruger from comment #5)
> So the situation has not changed yet

No, no change has been made neither upstream nor in openSUSE so far.

So currently, the SDDM settings module only reads /etc/sddm.conf, and that's
empty now by default so the upstream defaults (breeze with blue background) are
used.

There seems to be a separate problem with the handling of the background
setting though, at least with respecting the theme's default. (I'm not sure at
the moment whether it does read an existing theme.conf.user correctly, in
particular if you switch to a different theme)