> This is also popcorn that my coworker keeps inside or under his desk,
> so she is going into his personal space.

This isn't your battle to fight. Regardless if you bought the popcorn or not. If you gave it to your boss and Karen steals it from him, then she's stealing from him, not you. Let him deal with it if he's so inclined.

> She went into my desk to steal a half-eaten chocolate bar last year
> while we were going through a global pandemic. (Our boss is a big
> chocolate fiend and I guess he asked her if there was any chocolate
> while no one else was here).

It sounds like your boss instigated this. Surely, he shares some of the blame. That being said, I probably wouldn't put up much of a fuss over a half-eaten chocolate bar.

> She also doesn't pay people back whenever they purchase her food or
> snacks.

This isn't your concern, unless she borrows money from you and doesn't pay it back. Then your concern only goes as far as the money you lent her. What she does or doesn't do about money she borrows from other people isn't any of your business. You're not the office money lending police. Stay out of other people's business.

> This woman will be 59 years old on Friday. She is too old not to know
> better.

How is her age relevant? How do you know she's too old to not know? Maybe she wasn't raised with the same social norms that you were. You're making a judgment based on your own cultural/social bias.

> How do I politely tell someone with this much audacity to stop taking
> everyone's food?

You talk to her about stealing your food. What she does or doesn't do with other people's food is none of your concern. The other people can defend themselves if they're so inclined. You're not the office food police. Stay out of other people's business. Speak to Karen about taking your food and ask her to please stop doing it.