What is the theme of Mark Twain's "The Californian's Tale"?

There are several possible themes to Mark Twain's "The Californian's Tale."  It is a sweet and touching story of a man who lost his beloved wife years before, and each year expectantly awaits her return.  The narrator, a traveller in the parts that eventually settles in the area discussed in the story, comes across this man and slowly discovers his sad delusion throughout the course of the story.

A major theme that is presented is that of grief.  Nineteen years before the narrator met Henry, the man in the house, his wife had been captured by Indians on her way home from a trip.  After that incident,

"Henry lost his mind. He thinks she is still alive. When June comes, he thinks she has gone on her trip to see her parents. Then he begins to wait for her to come back. He gets out that old letter. And we come around to visit so he can read it to us.  On the Saturday night she is supposed to come home, we come here to be with him. We put a sleeping drug in his drink so he will sleep through the night."

So, Henry deals with his grief in an unusual way, through a denial of her death that surfaces once a year.  His kind neighbors are aware of his tendency, and come to keep him company and help him out.  Grief runs as a theme throughout the story.

Another theme is that of settling the west through gold mines.  The narrator opens by describing a gold town that had been deserted after the gold ran out.  That is how many towns and cities in the west were settled, through gold rushes and the people and trade they brought with them.  One last theme present in the tale is the roughness of the frontier west.  The narrator is amazed and awed at the little house that looks so clean and civilized, because he had

"been living for weeks in rough mining camps with other gold miners. We slept on the hard ground, ate canned beans from cold metal plates and spent our days in the difficult search for gold."

Very little civilization, niceties, or comforts were to be found in the frontier west.  Women were rare too, and the dangers were extreme, as was evidenced in Henry's wife's kidnapping by Indians.  It was an interesting time period, and Twain relates many aspects of it in his short story.  I hope that helped; good luck!