[backport cloud/1.38] Update manifest.json to remove color properties (#8613)

Backport of #8612 to `cloud/1.38`

Automatically created by backport workflow.

Co-authored-by: Alexander Brown <drjkl@comfy.org>

manifest.json

@@ -10,7 +10,5 @@
       "type": "image/svg+xml"
     }
   ],
-  "display": "standalone",
-  "background_color": "#172dd7",
-  "theme_color": "#f0ff41"
+  "display": "standalone"
 }