body {
    background-image: url("image.png");
    background-size: 31px 37px;
    background-position-y: -16px;

    min-height: 100vh;
    margin: 0;
    display: grid;
    grid-template-rows: auto 1fr auto;
}

header {
    min-height: 50px;
}

footer {
    position: fixed;
    top: 90%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 100%;
    text-align: center;
}

div {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    text-align: center;
}