Edit style.css and findout following

HTML Code:
body {
    background-attachment: fixed;
    background-image: url("../images/main_bg.jpg");
    background-position: center top;
    font-size: 12px;
    margin: 0;
    padding: 0;
}

Replace with

HTML Code:
body {
    background-color: #000000;
    font-size: 12px;
    margin: 0;
    padding: 0;
}