The theme of Hemingway's short story "Cat in the Rain" is isolation, public and personal isolation. George and his unnamed "American wide," also called "American girl" by the narrator, come to vacation in Italy near the sea, by a "public garden," and by a "war monument."
The narrator makes the point that Italians come from great distances to see the war monument and that artists like to paint and draw it; yet George and his "American wife" seem almost stationary in their hotel room. Granted, it is raining, but the contrast to the Italians and the stasis with which Hemingway opens and closes the story, with George unmoved and in the same position, gives a sense of permanency to their indifference and isolation from public gardens, war memorials, and refreshing sea breezes.
It is this mood of stasis that represents public isolation: George and the girl are not involved in the activities of public living. Even the hotel owner acts on the wife's behalf by sending a maid after her with an umbrella when she fails to be involved enough to ask for one on her own behalf.
Their personal isolation is best illustrated after the wife comes back in from trying to rescue the kitty in the rain (symbolically representative of the "American wife" herself). She starts talking about wanting the kitty and other tangibles since she can't have any fun, which is symbolically representative of happiness:
"Anyway, I want a cat," she said, "I want a cat. I want a cat now. If I can't have long hair or any fun, I can have a cat."
The narrator makes the point that George "hadn't looked away from her since she started to speak," saying, "You look pretty darn nice to me." This is the pivotal line of the short story, the line that points out the theme. She is so isolated that she can't recognize George's overtures of emotional intimacy, i.e., undivided attention and admiration. Once she resumes her litany of desire for the kitty and for material objects that replace involvement, he shows himself to be so equally isolated that instead of exerting himself to break through their isolation, he turns away and resumes reading a book, advising her to do the same: "Oh, shut up and get something to read."