GDM Background Image and Theme

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Hi, NixOS noob here.

Can you clarify what your patches do exactly? Does your first “very simple overlay” solution change the login background, or just the desktop background?

I want to change the login background. In hopes that that’s what you’re doing, I tried following what you did, but it didn’t work.

I copy-pasted the following into /etc/nixos/configuration.nix:

nixpkgs = {
  overlays = [
    (self: super: {
      gnome3 = super.gnome3.overrideScope' (selfg: superg: {
        gnome-shell = superg.gnome-shell.overrideAttrs (old: {
          buildInputs = (old.buildInputs or []) ++ [
            # CHEEKY WALLPAPER DERIVATION HERE
          ];
          patches = (old.patches or []) ++ [
            (pkgs.substituteAll {
              backgroundColour = "#d94360";
              backgroundPath = wallpaper.gnomeFilePath;
              src = ./greeter-background.patch;
            })
          ];
        });
      });
    })
  ];
};

and ran nixos-rebuild switch, and got the error error: undefined variable 'wallpaper' at /etc/nixos/configuration.nix:103:34.

Also, the next line says src = ./greeter-background.patch. What is greeter-background.patch? Is it the diff you placed just above? And where should I place this file for this to all come together?

Any help? Can you condense your instructions down to minimal steps that someone who has never patched NixOS can follow?