#background {position:fixed; z-index:0; width:100%; height:100%;
}

#centered {
position: relative;
top: 0;
right: 0;
margin: 0 auto;
width: 800px;
text-align: left;
z-index:1;
}

#scroller {position:absolute; width:100%; height:100%; top:0; left:0; overflow:auto; z-index:2;}