[YankForum]

you need to define "above_body" and "body_wrapper" seperately but rite now this doesn't work for you because earlier in your CSS you have defined this :

body {
margin: 0px 35px 0px 35px;
....
}

remove that and add fixed width to body_wrapper just like:

.body_wrapper {
width:900px; //or any other value
//and center it
}

and you should be done , but if header is not 100% then edit this above-body and define width:100%