The Wallpaper of the SDDM screen are always on day-mode, reducing consistency of the desktop. On some distros the wallpaper is also not changeable, so respecting that would be very handy.
Yeah, SDDM has no code to implement this. Right now all you can give it is a single wallpaper image file; no fancy day/night color scheme-aware wallpapers will work as they do elsewhere. This would need to be implemented in SDDM itself, which is not a KDE project. Please request it at https://github.com/sddm/sddm/issues. Thanks!