Answer:
I believe the theme is that greediness does not pay.
Step-by-step explanation:
According to the narrator, as long as the little grey mouse was respecting his food and space, everything was going well. But when the mouse became too greedy, when he not only ate all the food but also left a mess behind, the narrator decided to not put up with it. The mouse ends up caught in trap set by the narrator. If the mouse hadn't become greedy, he would have continued to live peacefully, easily, and freely.