style-wide.css, style-box.css

style-wide.css

body { background: #fff; }
#page { width: 100%; background-color: #fff; }

style-box.css

body { background: url(...png), no-repeat; }
@media (min-width: 1200px) {
    #page { width: 1170px; margin:0 auto; background-color: #fff; border-left: 1px solid #ccc; border-right: 1px solid #ccc;}
}