@claydoh is correct. There appears to be two relevant options:
- Appearance & Style > Colors & Themes > Login Screen (SDDM)
- Click the photo icon on the theme you want to change the background to when logging in for the first time (not yet logged in)
- Security & Privacy > Screen Locking
- Click Appearance > Configure… to choose a new wallpaper for your user’s lock screen (after you’ve logged in)
I guess the reason these are separate is because SDDM is for all users, when no user is logged in yet, and screen locking is per user (once you are logged in) so they technically should be separate settings.
In its current state, it’s pretty confusing on which setting controls what. At least I found it confusing when I updated the SDDM theme but noticed that the wallpaper was different than what I selected when locking my device. I could see this being improved in one of two ways:
- Add a tooltip or text or something explaining the difference and where to find the other option
- e.g., in the SDDM theme settings page, add a note like "This changes the wallpaper of the display manager only and does not affect any logged-in users’ lock screens. To change your lock screen, see [link to Screen Locking].
- Possibly combine these into one setting
- May not be appropriate if a user wants to use an alternative to SDDM
- Perhaps when setting an SDDM wallpaper, a prompt will ask if you also want to set the lock screen too.