lion/packages/babel-plugin-extend-docs/demo/my-extension/MyExtension.js

import { html, css } from 'lit-element';
import { MyCounter } from '../src/MyCounter.js';

export class MyExtension extends MyCounter {
  static get styles() {
    return [
      super.styles,
      css`
        button {
          background: #c43f16;
        }
      `,
    ];
  }

  _renderHeader() {
    return html`<h2>I am MyExtension</h2> `;
  }
}