An easy way to achieve this effect is to use a tiled image to span the width of the body. You can apply this in the CSS to the body element like this:

body{
background:#color url(body-header-tile.gif) scroll repeat-x top;
}

When you login to the site though this won't line-up due to the admin panel dropping the content down, so you may want to wrap all of the HTML in a wrapping div with a unique ID and apply the tile to that element instead.

Hope this helps.

Rick.