Are there a minified/compressed versions of the jQuery UI CSS themes on Google's CDN?

No, it does not seem to me, but I would advise you to ask this question on the dedicated group

Anyway, the provided versions seems already well optimized...

They don't have the minified version. Neither does the microsoft CDN have it. You wouldn't really need to compress it. If you really wanted to load faster, you need to include a custom package built on the jquery UI page to only include the features you need.