Update: The site no longer exists, instead would recommend you to try out DW Question Answer plugin which can be demo here — Question Answer Demo
After searching and using many question answer themes that are out there, I frankly didnt find any of these so called premium theme up to the mark. Then I came across this particular site which is also running on wordpress and has a theme of it own, here is a link to the site and below are the screenshots of it.
You can get this theme by contacting here
This is definitely the best question answer wordpress theme present out there ! Cheers !