body {
    display: flex;
    flex-direction: column;
    align-items: center;
    font-family: Arial, Helvetica, sans-serif;
}

#container {
    width: 960px;
    height: 960px;
    display: flex;
    flex-wrap: wrap;
    border: 3px solid black;
}

.square {
    box-sizing: border-box;
    border: 1px solid #eee;
}