.no-js #loader { display: none;  }
.js #loader { display: block; position: absolute; left: 100px; top: 0; }
.se-pre-con 
{
  position: fixed;
  left: 0px;
  top: 0px;
  width: 100%;
  height: 100%;
  z-index: 9999;
  background: url(loader.gif) center no-repeat transparent;
   
}

.modal-dialog {
 
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  
}