CSS (and Javascript) can be optimized in several ways:
- Minification - this compresses your CSS by removing whitespace, line breaks, and other non-functional parts
- Bundling - this merges several related CSS files into one, which reduces the number of requests made to the server
And more broadly you should ensure that your webserver is configured to support compression on your webserver, as well as at least a basic caching policy for your static files. These will help browsers be efficient.