I got it to work. Back-up your theme before doing theme edits!!
There are 2 CSS files to edit. How are you with editing the CSS files? The 1st change is line 118 in styles.css located in the root folder. Replace the original code of:
body {background-color: #000;}
With this code:
body {
background: url("yourimage_path") repeat scroll 0 0 transparent;
color: #7F7F7F;
font-family: helvetica,arial,sans-serif;
font-size: 12px;
height: 100%;}
Now the tricky part! In the CSS folder you will find dynamic.css. Locate line 74. Replace the code:
body #page, .sf-menu li, #primary-nav ul.sf-menu a:focus, .sf-menu a:hover, .sf-menu a:active, .commentlist ul.children .even, .alt #commentform textarea{background-color:#FFFFFF;}
With the new code of:
body #page, .sf-menu li, #primary-nav ul.sf-menu a:focus, .sf-menu a:hover, .sf-menu a:active, .commentlist ul.children .even, .alt #commentform textarea{background-color:#ffffff; width:762px; margin-left: auto; margin-right: auto;}
The width may need to be adjusted. I’m just working with a birds eye.. for perfect display, try it on yours. Adjust the width to the size you want… As for a border.. well add it to the second section code.. If you need more help, let me know.
No idea if you can work with CSS files.. always back up your originals before playing – ok.