html,
body {
    font-family: sans-serif;
    height: 100%;
}

html {
    display: table;
    margin: auto;
}

body {
    display: table-cell;
    vertical-align: middle;
}

body {
    background: #505050;
    color: white;
    text-align: center;
    padding: 50px;
}

button {
    background: white;
    color: #505050;
    border: 0;
    padding: 10px 20px;
    margin-top: 20px;
    cursor: pointer;
    outline: none;
    -webkit-transition: all 0.2s ease;
    transition: all 0.2s ease;
}

button:hover {
    background: black;
    color: white;
}