#site_center{width:800px; margin:0 auto 0 auto; padding:0 20px 0 20px;}

/* all curved boxes get the bottom-left corner */
#rounded {position:relative; background-color:#fff;}
#tl {position:absolute; top:0; left:0;}
#tr {position:absolute; top:0; right:0;}
#bl {position:absolute; bottom:0; left:0;}
#br {position:absolute; bottom:0; right:0;}