Hello,
I have come to the conclusion that IIS is my problem at the moment as I have fallen back to prior version of PHP and mediawiki with the same result.
Currently I have the same situation on both IIS 7.5 and 8 when I navigate to my mediawiki site it will render fine and then I do a page refresh and it returns with no stylesheet. I then refresh again and all looks fine.
Looking in a fiddler session when the stylesheet fails to load properly I see the following header on the URL supposedly fetching the CSS.
Cache If-Modified-Since: Fri, 03 Oct 2014 00:18:06 GMT Pragma: no-cache
The problem seems that the file is cached so it wont refresh itself and it ends up being read by the browser as a bad MIME type and is an empty file resulting in the plain HTML.
I do not have any URL Rewrite rules in place and have the same behavior from local server host and various remote machines under different browsers so its something being served back.