CSS is an html addition ... Search engines arent built to interact with the site, like users Google said they will also be indexing JavaScript and AJAX and CSS, so don't use it to hack.

All the search engines, except for one, I believe (but I forgot if it was Ask.com or MSN) said that you should not block your CSS and JavaScript files from the search engines using your robots.txt ...

Here’s how you can use this tool to fulfill your SEO robots.txt needs ... XML Sitemap Declarations and User Agent Disallow checking sections, you will see: CSS and JavaScript Resources are listed ...