It's possible to steal items at any time of day, in any situation. All you need to do is pick up a basket, bucket, kettle, or any other "container" type item and place it over any NPC's head. With the item over their head, they will not be able to see you as you plunder their Soon-to-be-bankrupt shop. This trick does not, however, work for pickpocketing. It only works for stealing items off counters, shelves, etc. Lastly, there are a few, select NPC's that will not allow you to put the bucket, basket, etc. over their heads. It seems like they have a sort of 'bubble' around them that prevents it from fitting on their head for more than a few seconds.