The easiest (and W3C compliant way) to center your site is to create a 'container' <div> like this:
<div class="container">
And then put this in your CSS file:
.container {margin: 0 auto; width: 950px;}
You'd then add the background color to the .body class which would fill in the sides next to the centered 'box'.
rajdeep, what is wrong with their signature? The full X-Cart version is there?