for a medium post

theme.direcive-snippet.ts

  ngOnInit() {
    this.themServiceSubscription = this.themService.observeActiveThemeChane()
      .subscribe(themeName => {
        this.themeName = themeName ;
        this.updateTheme(this.themeName);
      });
  }

  updateTheme(themeName) {
    const element = this.elementRef.nativeElement;
    const them = themes[themeName];
    for (const key in them) {
      element.style.setProperty(key, them[key]);
      this.document.body.style.setProperty(key, them[key]);
    }
  }