The light from the extension chord no, not enough power to do anything. but to play it safe just stick some black elec. tape over it.
Gaps from the door YES. You need to get some weather stripping and seal those cracks up.
i have a black tarp over the window and the room is always pretty dark but theres a couple of small holes in it so theres a little bit of light coming in.i have got my plants in the closet with the door slightly open pretty much all the time.would this disturb the dark cycle?