Conditionally add class to :host{} element via @HostBinding

add-class-to-host.scss

:host {
  background-color: white;
}

:host(.change-bg) {
  background-color: red;
}

add-class-to-host.ts

export class AddClassToHostComponent implements OnInit {
  // Behavior driving property
  conditionalProperty = false;

  @HostBinding(class.change-bg) changeBg = false;

  constructor() {}

  ngOnInit() {
    if (this.conditionalProperty) {
      this.changeBg = true;
    } else { this.changeBg = false; }
  }

}