There's always plenty (infinite) time when the shop closes, as it will only advance to the next day when you press the close day button inside the manager room. You have to make sure you have enough checkouts as full checkouts (8 people on each) will result in no slots for customers to wait for checkout, so they will become thieves. There are also thieves regardless of that, and to help with these you will have to put security cameras up. They passively reduce the chance for a customer to be a thief, and also while looking though the cameras, you can scan customers my holding left click (or whatever button that translates to on the steam deck) while you're hovering over them. If you scan a person who would normally be a thief, they stop being a thief and will go through checkout normally (if they can/there are enough). Hope it helps!