This page links to source code of XMLUI themes that were made publicly available by DSpace users. You can use these themes to get inspiration on what you can do in XMLUI and you can look at the source to see how you can do it.
The prefered way of sharing themes is now via GitHub, but feel free to add a link to locally hosted sources if it's easier for you.
The Ohio State University Knowledge Bank
Description & screenshots:
Mirage customization HOWTO: http://wiki.lib.sun.ac.za/index.php/SUNScholar/Asset_Presentation