html, body {
    height: 100%;

}

html {
    display: table;
    margin: auto;
}

body {
    display: table-cell;
    vertical-align: middle;
    background-color: lightgray;
    font-family: Verdana, sans-serif;
    color: gray;
}

#main {
    padding: 5px;
    text-align: center;
}

/* Center images. */
header img{
    display: block;
    margin-left: auto;
    margin-right: auto;
}

/* Hover change image visibility. */
header img:last-child {display: none;}
header:hover img:first-child {display: none;}
header:hover img:last-child {display: block;}

footer {
    padding: 30px 5px 5px;
    font-size: smaller;
    text-align: center;
    color: grey
}