How to grey out the background of a modal

I am using a modal in my app, the modal is not 100% wide nor 100% high. So I would like to grey out the view in the backgroud.
How to do this please ?

<ion-modal-view class="modal-center">

and

.modal-center {
  width: 96%;
  height: 96%;
  top: 1%;
  bottom: 1%;
  left: 1%;
  right: 1%;
  min-height: 0;
}

Thanks