https://csswizardry.com/2019/03/cache-control-for-civilians/#query-string–stylecssv1214

Query string caching is alot of time inafective for proxies and we whould use a fingerprint method instead. For example instead of 

stylesheet.css?3.2

it would be content.3.2.css or more preferably a hash of the contents stylesheet.ae3f66.css

Ultimatly this will also require finally moving to a merged css file. So the way it would work for style authors and admin would be to include the stylesheet.css file in the header and then include a custom.css file as well.

The backend should take each of these and automatically create a cached version with the appended fingerprint. Going forward with the new theme, it would work very similar the core.css file would ultimatly taken care of and fetched via a CDN

core.1.0.0.css then a themeName.css file would be included that would then get cahced by the backend with an appended fingreprint ie chameleon.ae3f66.css

The best way to handle this is probably a twig function that renders the proper include for the file based leveraging some sort of asset caching class or somthing alon those lines. it need not be more than maybe an 8bit hash.