If you reconsider the basic outlines of the plot, I think you will start to detect the theme of self-destructive obsession and fear of mortality that runs through Bradbury's story "The Skeleton."
Mr. Harris becomes obsessed with his skeleton, a symbol of death in cultures throughout the world. He regards...
his skeleton as separate from his true self, and it horrifies him:
"It's vulgar, it's terrible, it's frightening. Skeletons are horrors; they clink and tinkle and rattle in old castles, hung from oaken beams…"
Mr. Harris can't or won't internalize the reassurances of his wife and other people who lack his obsession. He eventually comes to believe that his skeleton is trying to kill him. He thinks it wants to free itself from Mr. Harris's flesh and Mr. Harris's control.
So Mr. Harris entrusts himself to a mysterious and self-proclaimed "bone specialist" -- a sinister little man who helped encourage Mr. Harris's obsession in the first place -- and this man responds by feeding on Mr. Harris's bones.
When Mr. Harris's wife comes home, she discovers her husband lying on the floor -- a human "jellyfish."
Did Mr. Harris get what he wanted? Obviously, existence without a skeleton is far more horrifying than existence with a skeleton -- even if you associate skeletons with death and gothic horror tales. If you strip away the surface details, and look at the underlying conflicts, you see a man who destroys himself because he can't reconcile himself to his vulnerable, animal, biological nature. If he had seen reason, and come to terms with the fact that bones are essential, he could have saved himself from ruin. But he was too much in the grip of his fear and horror, driven by his revulsion towards skeletons as terrifying, repulsive reminders of death and decay.
You can develop your own articulation of this theme by looking over the people Mr. Harris interacts with. Why don't other characters share Mr. Harris's obsession? His wife is clearly aware of the biological frailties of the human body. Note his conversations with her, and the fact that she works, or volunteers for, the Red Cross. Mr. Harris regards the fat man as someone who has successfully asserted control over his skeleton. What advice does the fat man give Mr. Harris, and how might Mr. Harris have benefited from it?
Remember that Bradbury could have made other choices, e.g., he could have given Mrs. Harris a different occupation. If you ask yourself why Bradbury made the choices he did, you will find more evidence to help you develop your own account of the theme.