Caffeine and theobromine, found in chocolate, can cause be fatal to small animals, such as cats and dogs, so I suspect it's not good for mice, either. It's a matter of body mass. Presumably, humans could die of chocolate poisoning, too, if we ate enough. (I think the fatal dose is roughly an ounce of chocolate per pound of body weight. A hundred-pound person would have to eat over six pounds of chocolate, which is unlikely.) Dark chocolate is a lot worse than milk chocolate, because it contains more theobromine, and baking chocolate is the worst.

Anyway...it's probably a good idea to remove the chocolate from her cage if there's any left.