body {
    font-family: Calibri, sans-serif;
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
}
header, nav, main, footer {
    width: 100%;
    padding: 1em;
    box-sizing: border-box;
    text-align: center;
}
nav {
    background-color: #7f7f7f;
    color: #fff;
    text-align: center;
}
main {
    display: flex;
    flex-direction: column;
    align-items: center;
}
@media (min-width: 600px) {
    main {
        flex-direction: column;
        justify-content: space-around;
    }
}