As it says: Styleyourforum is a place for style and code creators to upload their styles and codes.
You can go there and browse for stylesheets and codes to use on your forum.
When you find a style that you like the css code that you need to use, and possibly images for Customize Images are there on the page.
The codes that are listed there should say if they go in your header or your footer.
Directions for using stylesheets here: http://www.smfsupport.com/support/stylesheet_codes/how_to_install_a_stylesheet-t19069.0.html