switcher.ts

  constructor(@Inject(DOCUMENT) private document: any) {
    this.style = this.document.createElement('style');
    this.style.id = 'theme';

    this.document.head.appendChild(this.style);
  }

  initialize(): Promise<void> {
    this._theme = 'magister';

    return this.loadTheme(this._theme);
  }

  private async loadTheme(id: string) {
    if (environment.production) {
      return this.loadStylesheet(id);
    } else if (this.cache.has(id)) {
      this.style.innerHTML = this.cache.get(id);
    } else {
      return this.loadScript(id);
    }
  }

  private async loadScript(id: string) {
    const script = this.document.createElement('script');
    script.src = `/${id}.js`;
    script.onload = () => {
      const style = this.document.head.lastChild as HTMLStyleElement;

      this.cache.set(id, style.innerHTML);
      this.style.innerHTML = style.innerHTML;

      this.document.head.removeChild(script);
      this.document.head.removeChild(style);
    };

    this.document.head.appendChild(script);
  }

  private async loadStylesheet(id: string) {
    if (!this.link) {
      this.link = this.document.createElement('link');
      this.link.rel = 'stylesheet';
    }

    this.link.href = `/${id}.css`;
    this.document.head.appendChild(this.link);
  }