The Artisteer designs are applying background to the body so there is no way to push it down. Someone really good with CSS may be able to get that effect by wrapping everything in the body inside a div and applying the background to the div instead of the body, but that may throw off other things in the design.

You could apply a background color to the .topnav class so that the background image of the body doesn't show through.

Hope it helps,

Joe