Reload new stylesheets in-place.

Add a new optional parameter to resource.create, the URL of the resource to reload.
When creating a new stylesheet and this param is set, add the new element at the same position as the existing one.
You don't need the `appendChild` fallback. If the `prevEl` value is `null` or `undefined`, the `el` is just appended to the end of the list:
https://developer.mozilla.org/en-US/docs/Web/API/Node.insertBefore