Just create a file called
style.css.php (or whatever you prefer) in the same directory as your style file. Then add this code to it:
Now you just need to change the contents of any relevant <link> or <script> tags in your HTML to point to style.css.php instead of style.css and you're set! Feel free to play around with the expires offset – you might want to set it to 10 years in the future for long-term caching but make sure you change the name of the file whenever it's updated, otherwise the browser might not load the newer version.