body {
    font-family: Helvetica, "Trebuchet MS", Verdana, sans-serif;
    text-align: center;
}
header {
    color: blue;
    border-bottom-style: dotted;
    border-right-width: 20px;
    border-bottom-color: hotpink;
    margin: auto 300px;
}
header ul {
    color: green;
    list-style-type: none;
    padding: 0;
    padding-bottom: 10px;
    margin: 0;
    margin-bottom: 10px;
}
header a:hover {
    text-decoration: none;
    background: #cdfeaa;
}

#secret {
    background-image: url('doorclosed.png');
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center;
    display: block;
    margin: auto;
    width: auto;
}

#secret:hover {
    content: url('dooropen.png');
}