Hi David,
If you have strong knowledge about CSS you can create your own theme based on one of the existing ones. This document explains how to download/upload themes from MIME repository:
Then you can use WDTHEMEROOT Webdynpro application parameter to use your new personal theme.
Personally I think there is a new powerful tool for WebDynpro in the 7.3 release that can help you customize the application. The "HTML islands" that allow inject native HTML and CSS inline. Check this blog entry:
Regards,
Alex