html {
    margin: 0;
    padding: 0;
    background-image: url(images/clouds.jpg);
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
    background-attachment: fixed;
}
body {
    margin: 0;
    padding: 0;
    height: 100vh;
    display: flex;
}
section {
    margin: auto auto;
    text-align: center;
}
a {
    margin: 5px;
}
ul, li {
    margin: 0;
    padding: 0;
    display: inline;
    vertical-align: middle;
}