Answer:
The theme in Joan Didion's "Goodbye to All That" is: "It is foolish to expect a place that is not home to ever feel like home."
To allow this theme to emerge and develop, Didion contrasts how happy she was when she first lived in New York with the depression that hit her as the "magic" of the city wore off. She describes how she had initially romanticized New York and expected it to feel like home, but as time passed, she realized that it was just a place that she happened to be living in. Through her personal experiences and observations, Didion shows that it is unrealistic to expect a place to feel like home if it is not one's true home.