It could be that your server isn't sending CSS files back to the browser with the correct content-type header.
If you can configure your server to serve CSS files as stylesheets, instead of whatever it is that they are serving them as, it will fix this.
If it doesn't, do you have the Website URL field filled out on the settings page in the control panel?
If this still doesn't work, please login to our client area and post a support ticket.

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