Use a decorator for applyTheme

[tomivirkki]

Description

Currently, the following snippet is included in each Lit example. The snippet is verbose and not related to the component being demoed.

protected override createRenderRoot() {
  const root = super.createRenderRoot();
  // Apply custom theme (only supported if your app uses one)
  applyTheme(root);
  return root;
}

Consider wrapping the above functionality inside a decorator, which could then be applied to the Lit examples with:

@applyTheme // hidden-source-line
@customElement('button-basic')
export class Example extends LitElement {