You should not watch television in a darkened room because you will not see the picture so clearly. The human eye functions best and is able to see detail most acutely when viewing white light. So a lightish background makes for a better picture.
Of course, it is unwise to have the source of light shining directly on to the television set because some of the rays will be reflected and spoil the picture.
Viewing in a dark room can also cause an unpleasant glare. The intense light coming from a televsion screen against a black background causes a certain amount of stray light to be reflected internally from structures within the eye on to the retina. The retina is the delicate tissue containing the photo receptors.
The effect is similar, if less intense to that produced by a car with undipped headlights.
A third disadvantage of a darkened background is that "flicker" on the television screen becomes more apparent and can cause severe eye fatigue. In certain quite rare cases this "flicker" effect can produce epileptic fits.