The original UI is quite usable. When I started using Stack Over Flow, navigation was a headache as the links weren't clear. I'm accustomed to it now.

Why do you need a good navigation if people just hit the website to lookup the answer? If they want to become active members they'll learn it!

The font and colors choice are quite good for reading. Reading for long hours and long periods with this theme without getting tired or annoyed, I should say it was really well designed.