You'll notice that the grey gets lighter and lighter everytime you click closer to a discussion.
When reading and typing in a discussion we wanted the background to be white and when quoting a post it looks like a post it note... This is really easy on the eyes. For browsing the forum, the light grey sets us apart and goes with our overall theme. It's different.
I think it's better then very dark colors and a nice change from the default blue, that's just boring, lol we're the default grey, lol
In keeping with a professional tone, white, grey and black are the only colors.