Start your CSS files with this PHP (and name it style.php):

<?php
    ob_start ("ob_gzhandler");
    header("Content-type: text/css; charset: UTF-8");
    header("Cache-Control: must-revalidate");
    $offset = 60 * 60 ;
    $ExpStr = "Expires: " .
    gmdate("D, d M Y H:i:s",
    time() + $offset) . " GMT";
    header($ExpStr);
?>

body { color: red; }

Then call your CSS with the PHP file name:

<link rel='stylesheet' type='text/css' href='css/style.php' />