The themes of "Sredni Vashtar" are wishes and faith. The young boy is sick and doesn't have any agency or happiness; he knows that he's likely to die soon. He places his faith in a ferret that he begins to revere as a God. When his guardian wrongs him, he wishes that the ferret will take revenge on her. His faith is rewarded when the ferret kills her and the young boy appears happy with the result.

The tone of the story is serious. Even though a ferret isn't a God, the story is still told with gravity as if it could happen in the real world. It's also somewhat cold and dark; it mirrors the world the young boy lives in. Even though it's told from his perspective, the diction of the story seems to be from an adult's perspective. It uses terms that an adult would use—not a child.

To me, the major theme of this story is that people have a great capacity for acting in evil ways.  I think we can see this in both Conradin and his guardian, Mrs. De Ropp.

The guardian treats Conradin very badly throughout the story.  She is supposed to be his parent figure yet she delights in making him feel bad.  She feels triumphant whenever she denies him something that he wants.

On the other hand, Conradin is quite bad in that he wants such horrible revenge on Mrs. De Ropp.  He does rituals to try to keep her toothache going and he prays for her death.  When she is killed, he is happy.

So the theme to me is that people are cruel and evil.