Well as you will find in many posts, x-cart is highly customizable, but can cause a lot of sleepless nights. To answer your short question about the background color, you can edit the skin1.css file to change the body property.
Code:
BODY {
MARGIN-TOP: 0 px; MARGIN-BOTTOM: 0 px; MARGIN-LEFT: 0 px; MARGIN-RIGHT: 0 px;
BACKGROUND-COLOR: #FFFFFF;
}
Is what is in mine, the background-color controls the color. You should read up on css before making sweeping changes. You cant really paste code into xcart and have it work, there is a lot of smarty and java thrown in that will stop your cart from working if you just copied it over from dreamweaver.