You could ask HR what the consequences for theft from a colleague are. If there is no HR team then the company owner is HR. In many places and many countries the consequences would be dismissal.
Depending on the answer you get, you may pay Karen a visit, and tell your that according to HR, the consequences of theft will be getting fired. And that if you get evidence of someone stealing popcorn, you will hand that evidence to HR.
That makes it quite clear to her what you think, and she can't exactly go to HR and check. She also can't complain about it, as long as you don't say you suspect her. It doesn't necessarily have to be the truth, as long as HR told you they would take this serious.
PS. Some comments said that theft from someone else isn't your business. It is. With a thief in the office, nobody is safe.