Citadel

The Groupware Server for Web 2.0

Citadel is a complete and feature-rich open source groupware platform.

Email, calendaring/scheduling, address books.
Bulletin boards, mailing list server, instant messaging.
Multiple domain support.
An intuitive, attractive AJAX-style web interface.

Themes

The appearance of the web interface, WebCit, can be changed easily by using a theme.
The default WebCit installation will create an empty directory called static.local.
Easy install will do so in /usr/local/webcit/; the Debian-version in /usr/share/citadel-webcit/

Just untar the theme file in this static.local directory.
Once you've untarred the file, take a look at the webinterface.
If you don't like it, erase the files you just installed and you've got your standard WebCit looks again.

My first theme is Blue Citadel.
You can download the latest version of the theme file here.