angles are your friend, angles. So, this door opens inwards huh? and obviously can't have anything sticking on the exterior of the door. So, the best idea i have is to add a slab of wood over the seem of the door and the door fram on the inside, attached to the door. so, light wouldn't just go straight through the crack, it would have to go around that sheet/slab. Next, i would get place another sheet on top of the added sheet, so you have stairs in effect, and, with the now gap in between the adjacent wall and the added slab, i would put in foam window/door sealer(the stuff with the adhesive on one side) that should be light proof as a mug.