I've customized the interface (branded skin) in previous versions of plesk however the Interface Manage link takes me to the same page as the Languages. I cannot see how to upload a custom interface.
pls use search next time there are several threads about this.
skin support is not integrated in plesk10 at this time. but it will come in one of the next updates: http://forum.parallels.com/showthread.php?t=106628#8