Style core's progress bar

Drupal core's progress bar is unstyled.

You can see the markup for this using the styleguide module (D8 only) https://www.drupal.org/project/styleguide

My thought is to add 2px border radius, and make the border color and the "progress" part blue.

Be sure to extend core's library so the CSS only loads when necessary. Refer to Claro's info.yml file if you need to see how to do this