To have a just plain .css file get parsed by PHP, you need to tell Apache to parse CSS as PHP content. You can do that in the line in your httpd.conf file that looks like this:
<IfModule mod_php4.c>
AddType application/x-httpd-php .php
AddType application/x-httpd-php-source .phps
</IfModule>
Add:
AddType application/x-httpd-php .css
You may also be abel to use a .htaccess file to set this. A better solution might be to embed your CSS in your webpage with <style></style> tags...
Otherwise, POST CODE so we can see what doesn't work about your trial...