You obviously were very uncomfortable about being with these girls when they stole nail polish from the pharmacy. Perhaps you felt as if you were also guilty of stealing since you saw them do it and said nothing. You do not approve of stealing, but your friends were not bothered by it and may have stolen before.
I think that you need to ask these girls if they have stolen things before. Ask them why they steal, if they believe stealing is wrong, and if they plan on stealing anymore. Based upon their replies, you need to decide if you want to remain friends with girls who do not share your values about stealing. Can you simply ignore this part of who they are and continue to feel good about being friends with them? Will you be able to go into stores with them and watch them steal things without feeling guilty and like a thief yourself? Rewarding, long-lasting friendships are built on mutual respect and honesty. If you cannot continue to respect these girls or trust that they will not steal again, I think that it will be very difficult for you to continue to value this friendship. This is not an easy decision but I think it's one that you will make based on the right principles. Thanks for writing.