*{
    margin: 0;
    padding: 0;
}

div
{
    display: flex;
    flex-direction: column;
    border: solid;
    width: 50%;
    height: 200px;
    border-width: thin;
    box-sizing: border-box;
    justify-content: center;
    align-items: center;

    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;
}
button
{
    width: 8em;
    margin: 2em;
    background-color: aquamarine;
    border-color: black;
}