1import { AlertController } from 'ionic-angular';
2...
3constructor(private alertCtrl: AlertController) {}
4...
5presentAlert() {
6  let alert = this.alertCtrl.create({
7    title: 'Low battery',
8    subTitle: '10% of battery remaining',
9    buttons: ['Dismiss']
10  });
11  alert.present();
12}

1let alert = this.alertCtrl.create({
2	title: 'Low battery',
3    subTitle: '10% of battery remaining',
4    buttons: ['Dismiss'],
5    cssClass: 'custom-class'
6});
7alert.present();