First off, let's revisit something we have touched upon already: http://httparchive.org/interesting.php#bytesperpage

That link shows that as far as web assets go, CSS is almost always the smallest in terms of file size. Therefore, CSS file size optimization, in general, is a micro-optimization. That does not mean we should abandon the pursuit of economical CSS but we should be aware that optimizing a single image on the same page would probably save more in terms of file size.

Secondly, the web server that will be serving the CSS will be (or certainly should be) using gzip. This will automatically compress the files over the wire for us, shrinking them to a fraction of their original size.