Hello, By default, SDDM ships a default theme which uses a picture from the Maldives as its background; this is in contrast to most other display managers packaged in Guix (at least GDM, lightdm, slim) for which we set the background to an image from the Guix-artwork project.
We should package our own SDDM theme to have offer a more integrated look out of the box! -- Thanks, Maxim