html, body {
    height: 100%;
}
body {
    background-color: #ffffff;
    margin: 0;
}
.flex-container {
    height: 100%;
    padding: 0;
    margin: 0;
    display: flex;
    align-items: center;
    justify-content: center;
}
.row {
    width: auto;
}
.flex-item {
    padding: 5px;
    width: 200px;
    height: 200px;
    margin: 10px;
    line-height: 20px;
    font-weight: bold;
    font-size: 2em;
    text-align: center;
}