body {
    background: #111;
    padding: 0;
    margin: 0;
}

body > * {
    min-height: 100vh;
    display: grid;
    place-items: center;
}

header > div {
    width: 50vh;
    height: 50vh;
    background: red;
}

main > div {
    width: 50vh;
    height: 50vh;
    background: blue;
}

footer > div {
    width: 50vh;
    height: 50vh;
    background: green;
}

main {
    background: navajowhite;
}

header {
    background: antiquewhite;
}

footer {
    background: aquamarine;
}