message: docs(styling): update example with extended style
  Change the example of derived class with additional styles, to a more
  efficient way.
diff: /**
  * @license
  *
  - * Copyright IBM Corp. 2019
  + * Copyright IBM Corp. 2019, 2020
  *
  * This source code is licensed under the Apache-2.0 license found in the
  * LICENSE file in the root directory of this source tree.
  @@ -14,11 +14,13 @@ class MyDropdown extends BXDropdown {}
  // We seem to have hit something similar to: https://github.com/codesandbox/codesandbox-client/issues/1935
  // Custom CSS to enforce `field-02` (light) style of the dropdown
  -MyDropdown.styles = css`
  - ${BXDropdown.styles}
  +MyDropdown.styles = [
  + BXDropdown.styles,
  + css`
  .bx--list-box {
  background-color: white;
  }
  -`;
  + `,
  +];
  customElements.define('my-dropdown', MyDropdown);
  