body {
    font-family: Arial, Helvetica, sans-serif;
    margin: 0;
    padding: 0;
}

#container {
    margin: auto;
    display: flex;
    flex-flow: column wrap;
    width: 640px;
    height: 640px;
}

#container > div {
    display: flex;
    flex: auto;
    width: 100%;
}

#container > div > div {
    border: 1px solid black;
    flex: auto;
    height: 100%;
}

.focus {
    background-color: aqua;
}

#reset-grid {
    margin: 8px;
}