You can get the whitebox.dsl extension from the repository and use that to customize your fluxbox style. Also here is a great page to help you make your own. It covers everything you will need to know to get started creating your own style.
Another option is if you like one of the current styles that comes with DSL but just want to tweak it, you can find these in /usr/share/fluxbox/styles and the backgrounds in /usr/share/fluxbox/backgrounds. When you have created your file, you should place it in /home/dsl/.fluxbox/styles and it will show up in the styles menu.