The background of splash screen can not be changed. (I mean the splash screen while loading KDE, not SDDM or GRUB) I can already change the background of desktop and lock screen. It will be good if there is an option to also change the splash screen. Reproducible: Always
There's 'Splash Screen' settings for changing whole splash theme - but i guess your wish is to be able to change plain color, wallpaper.. of the splash just like in desktop/lock screens
Just get a new splash screen, that's why we make it so you can get and switch to new Splash Screens.