body {
  margin: 0;

background-image: url("http://getwallpapers.com/wallpaper/full/b/c/0/235141.jpg");

.header {
    background-color: #f1f1f1;
    padding: 20px;
    text-align: center;
}

.topnav {
    overflow: hidden;
    background-color: #333;
}

.topnav a {
    float: left;
    display: block;
    color: #f2f2f2;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

.topnav a:hover {
    background-color: #0000;
    color: black;
}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

li {
  display: inline;
}

ul {
  border: 1px solid #555;
}