place under "<body>"
<style>
body {
background-image: url(paste url here);
background-size: cover;
background-repeat: no-repeat;
background-position: center;
background-attachment: fixed;
height: 300vh;
}
.cover {
background-color: aqua;
height: 50vh;
margin-top: 90vh;
padding: 20px;
}
</style>