How to set sddm background on login

I try to use sddm in nixos. The theme background and conf are saved in /nix/store which I cannot edit. And I try to use sddm.extraConfig to set a background, but I cannot find the document about which options I should use.

After logining in, I can change background by feh, but how about login page?

If you want to apply a theme, you need to install an SDDM theme in nixos (I don’t think there are any packaged at the moment apart from the defaults, you might need to do it yourself), and then set the sddm.theme option to the name of the theme.

I don’t think SDDM lets you change the background independently of a theme.

Thanks for your answer.

I think it is possible to add a new argument in sddm’s drv and let user use override to change the theme’s config, but I don’t know whether others want to have this feature.

That shouldn’t be necessary: sddm has an appropriate themes directory set, and AFAIK themes are how you’re supposed to configure this stuff. If you wanted to change the sddm derivation I think you’d just end up modifying one of the default themes, which seems worse than just providing the theme you want.