.mainHeader {
height: 100vh;
min-height: 40em;
background-image: url(https://wallpaperaccess.com/full/1566401.jpg);
background-size: cover;
background-position: center;
background-repeat: no-repeat;
background-attachment: scroll;
}

#footer {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 2.5rem;  
  }