Hi,.
1. In federated portal systems:
To enable a continuous end user experience and display all local and remote content with the same theme (the theme of the consumer), you should import the consumers' theme to all producers. You do not need to assign this theme to any users; however the same theme with the same ID should be available to render the content accordingly.
2. If you have the same theme(same id) available in both consumer and producer, then you will have to check urversion of themes of producer portal and backend sytsem. Please refer sap note# 1446099 for more details. Portal stylesheet version should be higher than backend application stylesheet.
Thanks,
Rajendra