nixos/xdg/icons: add fallbackThemes option

nixos/modules/config/xdg/icons.nix

@@ -13,6 +13,14 @@
         [XDG Icon Theme specification](https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html).
       '';
     };
+    xdg.icons.fallbackCursorThemes = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [];
+      description = ''
+        Names of the fallback cursor themes, in order of preference, to be used when no other icon source can be found.
+        Set to `[]` to disable the fallback entirely.
+      '';
+    };
   };
 
   config = lib.mkIf config.xdg.icons.enable {
@@ -25,6 +33,15 @@
       # Empty icon theme that contains index.theme file describing directories
       # where toolkits should look for icons installed by apps.
       pkgs.hicolor-icon-theme
+    ] ++ lib.optionals (config.xdg.icons.fallbackCursorThemes != []) [
+      (pkgs.writeTextFile {
+        name = "fallback-cursor-theme";
+        text = ''
+          [Icon Theme]
+          Inherits=${lib.concatStringsSep "," config.xdg.icons.fallbackCursorThemes}
+        '';
+        destination = "/share/icons/default/index.theme";
+      })
     ];
 
     # libXcursor looks for cursors in XCURSOR_PATH

nixos/modules/services/desktop-managers/plasma6.nix

@@ -245,6 +245,7 @@ in {
     systemd.services."drkonqi-coredump-processor@".wantedBy = ["systemd-coredump@.service"];
 
     xdg.icons.enable = true;
+    xdg.icons.fallbackCursorThemes = mkDefault ["breeze_cursors"];
 
     xdg.portal.enable = true;
     xdg.portal.extraPortals = [