The theme of this story may be somewhere in the lines of "live your life to the fullest, don't waste any opportunities".
Wharton uses irony to express and convey this idea of living life and being free. For example, the writer mentions that the woman in the story hadn't had the chance of people envying her wedding presents (while her husband was very sick and in bed rest) This meant that she was more worried about living her life and people knowing she was a newlywed than her husband actually dying.
The story describes how this woman felt life hadn't been fair with her and she only wanted to live life up to its fullest, regardless of her surroundings and what was happening to her.
"deer in headlights" is an idiom.
"like deer in headlights" is a simile, the word "like" makes it a simile.
