ng-custom-properties#random-theme.ts

@Component({
  selector: 'app-root',
  template: `<app-card></app-card>`,
})
export class AppComponent {
  // ...

  randomTheme(): Theme {
    const primary = this.randomHex();
    const accent = this.randomHex();
    return { primary, accent };
  }

  private randomHex() {
    return '#' + Math.floor(Math.random() * 16777215).toString(16);
  }
}