Recently Google Search Console sent email notifications regarding their inability to browse CSS, and JS files. While it is likely that nothing has changed on your site to spur the notification, Google is apparently ramping up their customer awareness efforts after they started rendering the full page back in October. If you received this message the fix is a simple edit to your robots.txt file to make sure you are not disallowing access to JS, and CSS files.
A screenshot of the email:
Simply follow the steps in the email.