Thanks @Chris, site is based on awesome work of discourse team you can find details at http://meta.discourse.org/.
CSS is inspired from lot of sites which are mentioned at https://meta.discourse.org/t/please-visit-our-discourse-forum-directory/3102/135 some from which I got inspired from are http://forums.hummingbird.me/ , http://incendiarylegion.com/, http://discuss.howtogeek.com/ all based on discourse and also has been greatly inspired by simple but beautiful site http://stephengacheru.me/
For this site, I have modified a background and few colors and effects. If you need any particular code or area do let me know. Or better you can also ask at discourse site where many experts will also help.
Main focus was on keeping the design to very limited number of colors like
4eb1ba and E9E9E9, rgba(34, 41, 48, 0) and rgba(255, 255, 255, 0.1) with varying alpha value from .1 to .9.