For changing the background, you may edit the style.css file located in 'templates/css' folder.

Findout following :

body {
background-color: #000000;
background-image: url("./images/body.gif");
color: #FFFFFF;
font-size: 12px;
margin: 0;
padding: 0;
}

then change the 'background-color' and 'background-image'