that's not true at all. search the forums for responses from the zimbra guys about customizing. while they don't encourage it, the license says something like as long as you keep the logos and copyright intact and visible on the pages specified, you can do anything else you like, including adding your own logos/images/layout and moving the zimbra logos/copyright.
if you're a network edition customer you can ask for a pdf that explains how to customize the screens, but those steps would be in contradiction to the opensource license. maybe a wiki entry would be good from someone who has sucessfully customized the open source version in accordance to the licenses. i'm hoping someone will come up with nice skins soon
