Using Themes in a Federated Portal Network 

If you are using custom made themes or have modified any of the out-of-the-box themes on the consumer portal, an administrator must manually import these themes to the producer portal.

You do not need to assign this theme to any user; however, the same theme with the same ID must be available if you want to render remote-based content on the consumer with the same look and feel as other local-based content running on the consumer portal.

See: Exporting and Importing Themes