It looks like your server is incorrectly sending CSS files over as PHP files. I used Chrome on my mac and this is what showed up in the error console.
cssResource interpreted as stylesheet but transferred with MIME type application/x-httpd-php. -- common.css
If i try to browse to the css file directly it doesn't load for me like normal and instead tries to download. I would contact your host and ask them to fix the web server to serve up CSS files correctly. It probably works on Firefox since they are ignoring the server and just assuming its css based on the .css file extension.