One theme of the story that directly connects to Ratan and the Postmaster would be the ability to dream of what can be from what is. This ability to transform reality into something conditional is what drive both characters. The postmaster's dream of leaving Ulapur and going back to Calcultta is what animates his very being. He endures life in Ulapur in terms of being isolated, getting sick, and biding his time in the hopes of getting transferred out of the small town. For the postmaster, his hope of leaving is what animates his being. Ratan dreams of living a life with the postmaster. This is what animates her condition in the world. An orphan, Ratan has no one and is living a rootless existence until she is taken in by the postmaster. Her desire to be with him, to nurse him back to health, to learn from him, and to wait on his every need is fed by the hope that she can remain with him. She is unaware of his dream and when she recognizes it, she asks to go with him. His dream essentially comes at the cost of hers.
The theme of individual aspiration is what motivates both characters. It is what drives them closer to one another, only to drive them apart. The ending is one in which the postmaster has found relative comfort in his dream, while Ratan's pain seems to be more having come so close to envisioning her own dream and then see it dashed by the postmaster's.