Missing crucial documentation for Progress? [React]

[embostan]

Hello,

I was trying to setup the circular progress component and could not understand why only the label and value text showed up. It turns out that the circle elements' CSS was referencing 2 undefined variables (--size and --thickness).

Unless I missed something, the documentation mentions them nowhere. So is it a bug that they were not defined? I imported the Progress component like described in the docs.

Thanks!

[segunadebayo]

Want to open a PR to document this? It fits into a "Styling Guide" section of the page