.background {
    background-size: 100% auto;
    background-repeat: no-repeat;
    background-position: center center;
    
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0;
    left: 0;
    z-index: -100;
}

.background-dark {
    background-image: url("../images/background-dark.jpg");
    background-color: black;
}

.background-light {
    background-image: url("../images/background-light.jpg");
    background-color: white;
}