将聚合物3.0元素整合到pwa-starter-kit中的正确方法是什么？

Question

将聚合物元素3.0（例如：纸卡和纸按钮）插入pwa-starter-kit视图之一（例如：my-view2.js）后，无法正确显示。插入的元素显示为与APP布局重叠，就像这张图片中的纸卡一样。

我试图更改元素导入的顺序和位置（例如，在不同的视图中，例如在my-app.js中）。

import { html } from '@polymer/lit-element';
import { PageViewElement } from './page-view-element.js';
import '@polymer/paper-card/paper-card.js';
import '@polymer/paper-button/paper-button.js';
class MyView1 extends PageViewElement {
render() {
  return html`
  ${SharedStyles}
  <section>
    <h2>Static page</h2>
    <p>This is a text-only page.</p>
    <p>It doesn't do anything other than display some static text.</p>
  </section>
  <section>
    <h2>Welcome</h2>
    <paper-card heading="Emmental" alt="Emmental">
      <div class="card-content">
        Emmentaler or Emmental is a yellow, medium-hard cheese that originated in the area around Emmental, Switzerland. It is one of the cheeses of Switzerland, and is sometimes known as Swiss cheese.
      </div>
      <div class="card-actions">
        <paper-button>Share</paper-button>
        <paper-button>Explore!</paper-button>
      </div>
    </paper-card>
    `;
  }
}

window.customElements.define('my-view1', MyView1);