Cant' find this file on my server

Hi guys, I wanna edit something on a website, but firebug shows me that for the change that I need are on this link/file:

website.com/application/css.php?request=application/themes/slipstream/theme.css&c=146

How do I reach that exactly ?

Thanks!

You should be able to reach it at

hmmm ... thanks for the info, still it's not wordpress. And the /application/themes/slipstream/theme.css is not the same file that I need.