html, body {
    height: 100%;
}
body {
    margin: 0;
}
.flex-container {
    height: 100%;
    padding: 0;
    margin: 0;
    display: -webkit-box;
    display: -moz-box;
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;
    align-items: center;
    justify-content: center;
}
.row {
    width: auto;
    border: 1px solid #5d5dc3;
}
.flex-item {
    background-color: #d6583f;
    padding: 5px;
    width: 20px;
    height: 20px;
    margin: 10px;
    line-height: 20px;
    color: black;
    font-weight: bold;
    font-size: 2em;
    text-align: center;
}