Yes, it is of course bad for your eyes. Watching TV in the darkness may let your eyes not see in the bright situation which indirectly stimulate the vision nerves. If you see things in the bright place, the vision nerve will give the normal reaction. In the darkness, the eyeballs may need a long time to adjust the dark situation. And the TV screen's bright change may stimulate the eyes. For a long time, the eyes may get short sighted. You'd better avoid doing this and open the light when watching TV.
No it isn't bad for your eyes to watch TV in the dark. Because a television is a cathrode ray tube with is in effect shooting light onto pixels. Therefore when you watch TV in the dark you are basically watching a light. Having an alternate light source in the room will make no difference whatsoever to your eyes, except if that light source is reflecting off the glass TV screen which would cause a glare. If this happens then your eyes will really strain because they are trying to adjust to see the light from the CRT. So, having another light on when watching TV could actually hurt your eyes.