Open file templates/Default/css/cs.css, at the top of this you can see block
PHP Code:
body
{
font-family: Verdana, Tahoma, Arial, sans-serif;
font-size: 76%;
background-color: #DAE0EE;
background-image: url('../img/bg.gif');
background-position: top left;
background-repeat: repeat-x;
color: #3E4A62;
margin: 0 0 10px 0;
padding: 0;
}
and change line
PHP Code:
background-image: url('../img/bg.gif');