About Creating ZCS Themes
From Zimbra :: Wiki
For information about ZCS 5.0 Themes, see the ZWC 5.0 Themes
You can create a theme as simply as setting two colors and their variants, or you make heavy modifications to the "chrome" or outlines of the user interface with extensive use of images. Information is provided about the details of how the theming mechanism works to enable you to create more advanced themes.
Note that while the theming mechanism is very flexible, we recommend in the 4.x version of ZCS that you limit yourself to just changing colors and fonts. While there is nothing preventing you from making more advanced themes, we cannot guarantee that these themes will survive upgrades to the framework as we are planning some changes in the near future that will shift some things around. So if you don't mind having to possibly go back to the drawing board at some time in the future, go ahead and play around all you like.
If you have any questions or themes you want to share with the world, feel free to post them on the Zimbra Forums.