Thanks to Ron’s network wide menu plugin, a common header and menu is now being created for the network.
The menu is created on Site ID 1 and is made available to all of the other sites on the network.
Additionally, I started to change the CSS for the XenForo installation. The header image and menu have been created so that the WordPress and XenForo side look the same.
Note: I have not tested MediaWiki installations to see how well they behave.