html, body {
    height: 100%;
    margin: 0;
}

body {
    position: relative;
}

h1, h2 {
    margin: 0;
}

#overlay {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0,0,0,0.5);
    z-index: 10;
    font-size: 100px;
    color: white;
    text-align: center;
}

body.hover #overlay {
    display: block;
}