With GZIP compression, server side, ONE file is the way to go for maximum performance and openness.
For years Google was not looking at CSS. I thought, why hide CSS in files Google does not read. If Google sees every detail of a website, they will have more confidence in their search results; and thereby a site might have a higher ranking. Now, of course, they do want to see CSS.
Unfortunately, at least for one of my sites, after Oct. 13th this openness is not good enough for Google!