I was under the impression that the new disqus uses an iframe
and therefore custom CSS can't be applied:
http://help.disqus.com/customer/portal/articles/526768-introducing-the-new-disqus-and-f-a-q-
For the most part, customization is different in the new Disqus because we decided to completely re-implement our commenting embed inside of an iframe. This iframe is hosted on disqus.com and, as such, the browser won't let your website apply styles to it using CSS statements.