* {
    box-sizing: border-box;
}

body {
    font-family: Arial, Helvetica, sans-serif;
}

#wrapper {
    width: 80%;
    margin-left: auto;
    margin-right: auto;
    border: thin solid black;
}

header {
    margin: 10px;
    padding: 10px;
    background-color: midnightblue;
    color: white;
}

nav {
    display: flex;
}

nav > div {
    text-align: center;
    flex: 1;
}

main {
    display: flex;
}

main > div {
    flex: 1;
    margin: 10px;
}

footer {
    background-color: midnightblue;
    margin: 10px;
    padding: 10px;
}

footer > p {
    text-align: center;
    color: white;
}

/* TABLET */
@media only screen and (max-width: 1024px) {
    #wrapper {
        width: 100%;
        border: none;
    }
}

/* SMARTPHONE */
@media only screen and (max-width: 510px) {

    nav {
        flex-direction: column;
    }

    nav > div {
        width: auto;
        margin: 5px;
    }

    main {
        flex-direction: column;
    }

    main > div {
        margin: 5px;
    }
}