It's simple if you have a bit of knowledge of url rewriting.
Write a .htaccess file in your root directory, It would look something like:
RewriteEngine On
RewriteBase /
RewriteRule ^customized\.css$ css\_generator\.php [L]
Now just create a file css_generator.php having content:
body{ margin:0px; font:9px/11px "Tahoma", Arial, Helvetica, sans-serif; color:#010000;
background:#f3f6e1 url(images/body_bg_1.gif) repeat-x 0 0}
Your html should look like:
Understanding what just happened.
- On page load when your browser will load customized.css, it will be redirected to css_generator.php
- the contents of css_generator.php will be also available as yoursite.com/customized.css
Hope this helps