There are some techniques for avoiding downloading files that haven't change, is well explained here:
https://devcenter.heroku.com/articles/increasing-application-performance-with-http-cache-headers#conditional-requests
and
http://www.speedawarenessmonth.com/mobile-application-performance-caching/

We should evaluate to support both ETAGS and Last-Modified headers natively in the Filepool to reduce data consumption.

This should apply to remote CSS files and any file downloaded using the Filepool (or files from inline styles)