During shows, all the lights backstage are turned off. Obviously, it would be very hard to do things like costume changes and prop wrangling in the pitch black, so we have little crew lights scattered about. And these crew lights normally have a blue gel over them to diffuse the light so it isn't insanely bright and jarring.

We even have blue flashlights for those really insane scene changes. There is definately truth to the phrase "theatre techs do it in the dark"