html,body {
    margin: 0;
    width: 100%;
    height: 100%;
    background: black;
    color: #eee;
}
#flex {
    display: flex;
    height: 100%;
}
form {
    margin: auto;
    display: table;
}
form p {
    display: table-row;
}
form p span {
    display: table-cell;
    padding: 0.5em;
}
form p > input {
    display: table-cell;
    text-align: center;
    margin: auto;
}