CSS Bootstrap Modal заменить затемнение на засветление

@import "//cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta/css/bootstrap.min.css"; 
 
.modal-backdrop { 
  background: #fff; 
} 
 
.modal-backdrop.show { 
  opacity: .8; 
}

<script src="https://code.jquery.com/jquery-2.2.4.js"></script> 
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js" ></script> 
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta/js/bootstrap.min.js"></script> 
   
  <!-- Button trigger modal --> 
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModal"> 
  Launch demo modal 
</button> 
 
<!-- Modal --> 
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true"> 
  <div class="modal-dialog" role="document"> 
    <div class="modal-content"> 
      <div class="modal-header"> 
        <h5 class="modal-title" id="exampleModalLabel">Modal title</h5> 
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"> 
          <span aria-hidden="true">&times;</span> 
        </button> 
      </div> 
      <div class="modal-body"> 
        ... 
      </div> 
      <div class="modal-footer"> 
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button> 
        <button type="button" class="btn btn-primary">Save changes</button> 
      </div> 
    </div> 
  </div> 
</div>