Adding graphical border styles to a fixed width container is simple. You create a 900px image, add your left and right styles to it and save it. (It does not have to be a big image, a small one will do that you can repeat vertically)
In this example, you have a fixed width, centred container that repeats your left and right borders through the background image you have created. The left and right corners are 30px wide, so you deduct that value from the width.
width:840px; /* -30px left border and -30px right border */
padding: 0 30px 0 30px; /* make room for bg to show, 30px each side */
margin: 0 auto; /* center container */
background:url(mybackground.png) repeat-y; /* repeat bg vertically */
A much more flexible method, however, is demonstrated and explained here.