Creating a new theme is very easy. You need to describe window's look and feel in a CSS file. Name your CSS file THEME_NAME.css with thoses CSS definition:
By convention the images used in the CSS files are in THEME_NAME directory (it's just a convention, do it as you feel!)
A window is divided into 9 parts, 4 corners, 4 borders and the main content in the middle.

View a detail template CSS file (you can copy-paste it to start a new theme). See Art.Gnome.Org for cool designs. I used two of them, it's easy to integrate in the class.

Take a look to an existing CSS file like alphacube.css or mac_os_x.css (with PNG image and IE specific code)

Themes included in this release