Well going dark is initiating no contact at all and only responding to contact as minimally necessary to discuss financial issues. Going dark is really only possible if there are no kids involved. Going dim is basically going as dark as possible with the understanding that you have to maintain a certain level of contact because of the kids. Someone here used to say "keep all contact to bills and boys" which is kind of going dim in a nutshell. You've got to maintain contact to coordinate dropping off/ picking up/ school projects/ homework/ sporting events/ practices/ unexpected costs/ medical issues/ medication/ etc. etc. I know that in my case that is quite a bit of required contact, almost to the point where I don't know if I'd even call it "dim" in my case.
Some people say they are dim/ dark with their live-in WAS, but I just don't think it's possible when under the same roof.