The appearance of the Zimbra Web Client (ZWC) interface can be customized. The customization can be as easy as selecting which of the existing Zimbra themes are available for users to select. You can create a theme simply by setting two colors and their variants within one of Zimbra sample themes to change the background, or you can make more elaborate changes to the user interface using images in addition to changing colors.
The ZWC user interface theme that displays as the default on the Zimbra Web Client and the themes that users can select in the ZWC Options>General tab can be configured in Class of Services or in individual accounts. A number of Zimbra themes are included with ZCS.
Go to the Zimbra Wiki, and view the Zimbra Rebranding and Themes page for complete documentation about how to customize a theme and deploy it to your Zimbra servers.