feat: add custom SDDM login theme

nixos/hosts/nixos/default.nix

@@ -5,6 +5,7 @@
     ./hardware-configuration.nix
     ../../modules/desktop.nix
     ../../modules/hardware.nix
+    ../../modules/login-theme.nix
     ../../modules/packages
     ../../modules/system.nix
     ../../modules/users.nix

nixos/modules/login-theme.nix

@@ -0,0 +1,26 @@
+{ pkgs, ... }:
+
+let
+  theme = pkgs.stdenvNoCC.mkDerivation {
+    pname = "sddm-custom-environment";
+    version = "1.0";
+    src = ../../sddm;
+    dontBuild = true;
+    installPhase = ''
+      runHook preInstall
+      themeDir="$out/share/sddm/themes/custom-environment"
+      mkdir -p "$themeDir"
+      cp Main.qml metadata.desktop "$themeDir/"
+      cp ${../../wallpapers/night_city.jpg} "$themeDir/background.jpg"
+      cp ${../../wallpapers/kotost.png} "$themeDir/avatar.png"
+      cp ${pkgs.nerd-fonts.profont}/share/fonts/truetype/NerdFonts/ProFont/ProFontWindowsNerdFontMono-Regular.ttf "$themeDir/font.ttf"
+      runHook postInstall
+    '';
+  };
+in
+{
+  system.build.sddmTheme = theme;
+  environment.systemPackages = [ theme ];
+  services.displayManager.sddm.theme = "custom-environment";
+  services.displayManager.sddm.extraPackages = [ pkgs.qt6.qtimageformats ];
+}