Themes are a good way to visually differentiate and/or brand SharePoint sites. Here is how to install a new theme.
1. modify an existing theme
2. add preview image to 12\TEMPLATE\IMAGES
3. modify the SPTHEMES.XML file in 12\TEMPLATE\LAYOUTS\1033 folder.
4. IISRESET
References:
http://msdn2.microsoft.com/en-us/library/aa979310.aspx