@media (max-width: 767px) {
  
  .modal {
    position: absolute;
    top:   20px;
    left:  20px;
    right: 20px;
    width: auto;
    margin: 0;
  }

  .modal.fade.in { top: 20px; }

  .modal-body {
    overflow: visible;
    max-height: none;
  }

}

@media (max-width: 480px) {
  
  .modal {
    top:   10px;
    left:  10px;
    right: 10px;
  }

  .modal.fade.in { top: 10px; }
}