style.css doesn't refresh while Bootstrap sub-theming
The mangled file comes from aggregating CSS files. You will need to clear/flush the cache in order to see changes.

You can turn CSS caching off by going to /admin/config/development/performance and unchecking aggregate CSS files. Now you don't need to clear the cache to see CSS changes. 



PS: Remember to turn this back on when your website is ready for production/going live.