feat(forgejo): add option for custom forgejo theme

modules/forgejo/default.nix

@@ -41,7 +41,9 @@ in {
   };
 
   systemd.tmpfiles.rules =
-    lib.optional useSubdomain
+    lib.optional (!(builtins.isNull cfg.themes))
+      "L+ ${forgejo.stateDir}/custom/public/assets - - - - ${cfg.theme}"
+    ++ lib.optional (!(builtins.isNull cfg.templates))
       "L+ ${forgejo.stateDir}/custom/templates - - - - ${cfg.templates}";
 }
 // lib.mkIf cfg.createUser {

modules/forgejo/options.nix

@@ -35,6 +35,16 @@ args@{ config, lib, ... }:
         '';
       };
 
+      theme = lib.mkOption {
+        type = lib.types.nullOr lib.types.path;
+        default = null;
+        description = ''
+          A directory containing a custom Forgejo theme.
+
+          This directory should contain two subdirectories `css` and `img`.
+        '';
+      };
+
       templates = lib.mkOption {
         type = lib.types.nullOr lib.types.path;
         default = null;