Change opacity of component ionic-backdrop

hi guys, anyone know how it is possible to change the opacity of an ion-backdrop is displayed behind the alert? I have tried several ways but don’t get it :frowning:

Y try with

ion-backdrop {
--ion-backdrop-color: rgba(0, 0, 0, .9);

But opacity do not change


Finally i’ve do it using the cssClass property in alertController

  async alertError(message: string) {
    const alert = await this.alertController.create({
      cssClass: 'alertClass',
      animated: true,
      header: 'Error',
      buttons: ['OK']
    await alert.present();
 ion-alert {
     background: rgb(0,0,0,.8);