body {
    box-sizing: content-box;
    margin:0;
    padding: 0;
    background-color: #efefef;
    background-repeat: no-repeat;

}

.centered {
    text-align: center;
}

@media (min-width: 921px) {
    body {
        background-image: url(./3d_fullsize.jpg);
        background-size: cover;
    }

    .whensmall {
        display: none;
    }
}

@media (max-width: 920px) {
    body {
        background-image: url(./3d_mobile.jpg);
        background-size: contain;
    }

    .spacer {
        min-height: 500px;
    }
}

@media (max-width: 600px) {
    body {
        background-image: url(./3d_mobile.jpg);
        background-size: contain;
    }

    .spacer {
        min-height: 350px;
    }
}