Yes, it's bad to look at the screen in the dark. As we know, when you look at the screen in the dark, it will make your eyes strain to see. And this is probably why most people get tired while watching TV. You should know that the pupil in the eye is constantly adjusting in size to the different levels of light, so it could cause eye fatigue. In other words, if you do it in that way, your eyes have to focus on the point for a long time. In the end it makes your eyes tired and could make it temporarily harder to focus. Even if it's not likely to cause permanent damage, it also can damage your eyes. So just do not watch TV in the dark.
Of course, it will have bad effect on your eyes. When you watch TV in the darkness, the light source from the TV screen may be stimulate your optic nerve slowly because of the adaptability. Then when you optic nerve is accommodated, the nerve system will not that sensitive. Thus, if you keep doing this, the eyes may have the weak reaction in seeing things which cause you to be short sighted.