Preview:
import { DOCUMENT } from '@angular/common';
import { Inject, Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class ThemeService {
  constructor(@Inject(DOCUMENT) private document: Document) {}

  switchTheme(theme: string) {
    let themeLink = this.document.getElementById(
      'app-theme'
    ) as HTMLLinkElement;

    if (themeLink) {
      themeLink.href = theme + '.css';
    }
  }
}
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