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 blue;
}
.flex-item {
    background-color: white;
    padding: 5px;
 /*   width: 500px;
    height: 40px;*/
    margin: 10px;
/*    line-height: 20px;*/
    color: purple;
    font-weight: bold;
    font-size: 2em;
    text-align: center;
}