body {
        background: #eee;
}
html, body, .center-outer {
        margin: 0;
        min-height: 100vh;
}
.center-outer, .center-inner {
        display: flex;
        align-items: center;
        justify-content: center;
}
.center-outer {
        flex-direction: row;
}
.center-inner {
        flex-direction: column;
        text-align: center;
}