If a room or an object the room is in (going all the way up to @musicmud) is flagged as Dark then the room is dark.

If the room is Outdoors and it is in the hours of night-time, then the room is dark.

If the room is flagged as Light, then the room is light. An outdoors room which is Light will be assumed to have automatic artificial street-lighting, and this will be mentioned in 'look.

Any object, person, player in the room (including those in people's inventories and in their containers), will cause a Dark room to be light. If this is required, it will say in 'look' what causes the light.

In a dark room, you cannot look or exits. You also cannot refer to any objects in the room, because you cannot see them.

If you want say, a light switch to be usable in a Dark room, give it the Glowing flag. You will not be able to see it in look, still, but you can refer to it.