Do you mean border around your forums or just an image as your background?
If you mean just an Image as you background. go to
Modify Styles in the the body tag change where it says bgcolor=#somecolor to this
Code:
background="{ imagesfolder}/background.gif"
remove the space after the { before imagesfolder
And if you want your background image to be watermarked. which means to stay stationary and have your forums slid over the background without the background moving add
Code:
bgproperties="fixed"
before the code mentiond above. this is a nice effect if you make your Page Background Color Transparent
So your Body Tag will look like this
Code:
<body bgproperties="fixed" background="{ imagesfolder}/background.gif" text="#CCCCC8" id="all" leftmargin="10" topmargin="0" marginwidth="10" marginheight="10" link="#000020" vlink="#000020" alink="#000020">