code: private void injectAdditionalStyles() {
  
          try {
              Collection<String> stylesheets = OpenCms.getWorkplaceAppManager().getAdditionalStyleSheets();
              for (String stylesheet : stylesheets) {
                  A_CmsUI.get().getPage().addDependency(new Dependency(Type.STYLESHEET, stylesheet));
              }
          } catch (Exception e) {
              LOG.warn(e.getLocalizedMessage(), e);
          }
      }
docs: Inject external stylesheets.