hmm maybe if you encrypt it in md5 hash or something like passwords.. but it's just css lol why go to the trouble it only has names of colours and fonts in, hardly sensitive data worth protecting? listen to everyone above you can't do it! only prevent the time it takes to view it!
^can a browser unencrypt md5 hash? If it can't, same problem as having no CSS at all. Browser sees nothing.
Even if you prevent people from directly downloading the CSS file (by making it a server-side script and ensuring that the referrer is from your site), it still wouldn't work because once a visitor has viewed your site, the CSS file is located in their cache and can be viewed. As others have said, why would you need to do this? Seems to me that JavaScript code would be something more worthy of protection. There's a thought - have the CSS file be inserted via JavaScript, and just obfuscate the JavaScript code. Even that would be far from bulletproof. There is no bulletproof way to go about it.