docs: theme schema update

packages/web/public/theme.json

@@ -2,6 +2,10 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "type": "object",
   "properties": {
+    "$schema": {
+      "type": "string",
+      "description": "JSON schema reference for configuration validation"
+    },
     "defs": {
       "type": "object",
       "description": "Color definitions that can be referenced in the theme",
@@ -163,4 +167,3 @@
     }
   }
 }
-