header {
    background: url('../img/index.jpg') center center;
    background-color: #ffff;
    background-size: cover;
    min-height: 70vh;
}

#cielo {
    background: url('../img/cielo.jpg') bottom center;
}

#buildings {
  right: 0;
  bottom: 0;
  min-width: 100%; 
  min-height: 100%;
}