How to change the background of the login screen (sddm)?

When I change the background of the login screen:

System Settings → Colors & Themes → Login Screen (SDDM) → Change Background

it resets to the old background (see the demonstration here).

Is it a bug?

IMO that looks like a bug you could report to bugs.kde.org. I’ve not used the SDDM theming settings much but last time I did, it definitely didn’t randomly reset itself!

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I think I’ve found the relevant bug report, happens only when the name of the new background is the same as as the old one, the bug is 7 years old :grin:

Workaround is to check which name is in
/usr/share/sddm/themes/breath/theme.conf.user
(in my case it’s 5120x2880.png) then preplace the image in
/usr/share/sddm/themes/breath/5120x2880.png
with a link to the image you want to set as a background.

Or set a backround first to any image with a different name, then to the image you want.