Turn SCSS code into mixin

This would make it simpler to integrate into my codebase. I can submit a pull request for this if you could take care of updating the example page.

Start by locating the SCSS source and the example page in the repository. Review how the current styles are integrated, then determine the mixin interface and update the example page; done means the SCSS can be reused as a mixin and the example still renders the progress bar.

Written by the indexing model from the issue text.