найти:
body {
background: url({style_images_url}/bg.jpg) repeat-x, url({style_images_url}/bg_repeat.jpg);
color: #5a5a5a;
font: normal 11px tahoma, helvetica, arial, sans-serif;
position: relative;
padding-bottom: 20px;
}
заменить на:
body {
background: url(ссылка на фон) fixed;
color: #5a5a5a;
font: normal 11px tahoma, helvetica, arial, sans-serif;
position: relative;
padding-bottom: 20px;
}