In the 'body' styles in your common.css stylesheet there is a float-left attribute.
body {
background-image: url(../Images/body_bg.png);
background-repeat: repeat-x;
margin: 0;
padding: 0;
text-align: center;
float: left;
font-family: Arial, Helvetica, sans-serif;
}
Try removing it:
body {
background-image: url(../Images/body_bg.png);
background-repeat: repeat-x;
margin: 0;
padding: 0;
text-align: center;
font-family: Arial, Helvetica, sans-serif;
}