.ventana-modal {
  display: none;
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.5);
}

.contenido-ventana {
  background-color: white;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}