.site-background {
    position: fixed;
    top: 80px; /* height of your header */
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -1;

    background-color: #DDDFE6;
    background-image: url(images/bg1.png);
    background-position: center top;
    background-repeat: no-repeat;
    background-size: cover;
}