You need to give wrap1 a width so that margin auto works which also means removing the float.
Alternatively wrap another element around the page and use the width and margin:auto to center it.
background:red url(http://www.officelinkonline.com.au/test/images/shadow.gif) right bottom no-repeat;
margin: 0 auto;
The shadow is not showing at the bottom because you have absolutely placed .back on top of it. You need to move .back by 10px to let the shadow show.
If you want the bottom right corner to match your layout then re-make shadow.gif with a round corner!!
The same goes for the background colour. Re-make those image so that they match your background colours. You could use transparent pngs instead but IE6 doesn't understand transparent pngs and you would need to hack for it which gets complicated. The easiest solution is to make the images fit your layout colours.