Excess isn't a moral term.

What? What does that even mean? There aren't "moral terms." There's morality and ethics. Those are argued in normal language. For example, here is an discussion about a very similar topic from a Kantian perspective. It's just normal language.

The point is, stealing from the wealthy to feed the poor is, in my opinion, a moral good. I would say most people agree with this. That's why we have so many stories where this is what the hero does.

Which is problematic, as morality indeed is subjective.

I said it is, and you agree, but it's also important to keep in mind some people do subscribe to a system of universal morality.

Personally, I struggle to conjure a moral ground to steal food if I know where the food banks are.

But this takes limited resource from the food banks that they could use to feed others. I think, as a hypothetical, food banks should be able to take money from the wealthy to fund their work. They aren't allowed to do this, and it would negatively effect their efforts if they did it and weren't allowed, so I still see it as a moral good for people capable of taking from the wealthy to do so, before consuming limited resources that could help others.

Or if said food is of a treat / luxury category.

That partially depends on your definition, but that's fine. Is lobster a luxury? What about steak? What about a salad? What about bread? They're all providing sustenance. Why does the cost effect the morality? If it's moral to steal at all for food, where does cost come into that? Personally, I think people arguing that poor people should only get the absolute worst to survive aren't being considerate of those people. I think they deserve to eat something enjoyable sometimes.