The opening sentences of Saki's "Sredni Vashtar" establish the theme of survival in the story:

Conradin was ten years old, and the doctor had pronounced his professional opinion that the boy would not live another five years. The doctor was silky and effete, and counted for little, but his opinion was endorsed by Mrs. De Ropp, who counted for nearly everything.

In the first sentence, the reader learns that Conradin is unlikely to survive to adulthood. However, in the next sentence it is strongly implied that he could probably survive the doctor's adverse diagnosis were it not for the malign influence of Mrs. De Ropp. Her "coddling restrictions" have exacerbated his condition and lessened his chances of survival. Saki adds:

Without his imagination, which was rampant under the spur of loneliness, he would have succumbed long ago.

Conradin's imagination has allowed him to survive, and his worship of Sredni Vashtar is a central part of this life-sustaining imagination. When he believes that Mrs. De Ropp is certain to have her way and get rid of Sredni Vashtar, he gives way to despair:

And he knew that the Woman would triumph always as she triumphed now, and that he would grow ever more sickly under her pestering and domineering and superior wisdom, till one day nothing would matter much more with him, and the doctor would be proved right.

At its heart, the story is a struggle for survival between Conradin and Mrs. De Ropp. The savagery covered by a thin veneer of civilization is typical of Saki. The reader may think that Conradin bears some moral responsibility for his guardian's death, but it is equally clear that if she had survived, he would not have been able to do so.

What quotes relate to the theme of triumph in Saki's "Sredni Vashtar"?

When discussing quotes related to the theme of triumph, I think it would be useful to look into the battle of wills being waged between Conradin and his cousin. Even early in this story, Saki establishes Conradin and his guardian as being mutual antagonists to one another: note the story's second paragraph in particular, which does much to establish this sense of conflict. While these sentences do not directly relate to your theme of triumph, it is still important to note that, in order for someone to be victorious, they must first have some kind of obstacle or antagonist to overcome. In Conradin's case, it is Mrs. de Ropp, and in Mrs. de Ropp's case it is Conradin.

From here, you might cite numerous quotes within this story that reflect small triumphs and acts of rebellion on Conradin's part. In a sense, even his haven in the garden would apply, to say nothing of his invented religion surrounding the ferret (which could itself be understood as being a direct mockery of Mrs. de Ropp's Christian faith). On the other hand, there is also the other side to this struggle of wills, with Mrs. de Ropp's victories over Conradin. Consider, for example, the fate of Conradin's hen, or, in the end of the story, when she takes Conradin's key and goes down to the shed. Finally, and most importantly, there is the story's ending regarding Mrs. de Ropp and the ferret. In any sense, the examples are numerous, and there are a great many quotes you can choose from to express this theme of triumph in "Sredni Vashtar."