What are the major themes of the story "The Ones Who Walk Away From Omelas?"
The major themes of "The Ones Who Walk Away From Omelas" involve moral boundaries, rights of the individual vs. the collective, and the personal choice to justify a small evil for a greater good. The wondrous paradise of Omelas is explicitly said to be dependent on the suffering of a single child:
If the child were brought up into the sunlight out of that vile place, if it were cleaned and fed and comforted, that would be a good thing, indeed; but if it were done, in that day and hour all the prosperity and beauty and delight of Omelas would wither and be destroyed. Those are the terms. To exchange all the goodness and grace of every life in Omelas for that single, small improvement: to throw away the happiness of thousands for the chance of happiness of one: that would be to let guilt within the walls indeed.
(Le Guin, "The Ones Who Walk...," liferoar.wordpress.com)
The justification is that the child is too mentally damaged to enjoy any quality of life, and allowing it to escape its horrific prison would destroy Omelas, so it might as well be left there and ignored. This, of course, places the "guilt" which is mentioned and which is presumably entirely absent from Omelas, on every single citizen who knows of and allows the child's mistreatment; the child is even explicitly shown speaking at first, only to mentally degrade as time goes on. As a moral choice, it is unsaid but understood that the needs of the many outweigh the needs of the one, and so the condition of the child never changes.
The people of the title, who leave Omelas for some unknown place, are then the people who cannot justify for themselves the deliberate decision to hurt others for their own gain. They embrace individualism and remove themselves from Omelas, even though they do nothing to change the common state itself.
This story conveys the idea that most people are willing to enjoy prosperity and happiness at the cost of someone else's quality of life; the majority are able to find a way to justify the misery of one of the most vulnerable individuals—a child—if it means that they will never have to endure such misery themselves.
The fact that some few people are willing to walk away from this trade-off shows a couple of things: 1) the folks who will refuse the offer to exploit another person in order to secure their own happiness are in the minority, and 2) it is even more difficult to find someone who is willing to stand up and try to change the system that involves this exploitation. We see no one attempt to change the status quo; they may walk away and register their dissatisfaction this way, but why do they not actually go and release the child from its prison? Why not dismantle the system? It would only take one person to do so because, as the narrator says, it would happen in the moment that the child is brought out into the sunlight.