Merge pull request #138336 from shdpl/release-21.05

nixos/modules/services/x11/display-managers/lightdm.nix

···
       };
 
       background = mkOption {
-        type = types.path;
+        type = types.either types.path (types.strMatching "^#[0-9]\{6\}$");
         # Manual cannot depend on packages, we are actually setting the default in config below.
         defaultText = literalExpression "pkgs.nixos-artwork.wallpapers.simple-dark-gray-bottom.gnomeFilePath";
         description = ''