Different thickness for empty arc

Would it be possible to have a different thickness for the empty arc behind the filled one?

From an initial look at the code, it should be trivial to add a new parameter that defaults to `thickness` if not specified.

I can submit a PR if there's no potential blocker with this proposal. Are there any contribution guidelines?

Start by reviewing the repository's contribution guidelines and the existing handling of the `thickness` option. Confirm where a separate empty-arc thickness parameter belongs, default it to `thickness` when omitted, and verify that the empty arc renders with the requested thickness.

Written by the indexing model from the issue text.