Conversation so far:
What was Norman Rockwell's most famous painting?
Among the best-known of Norman Rockwell's works are the Willie Gillis series, Rosie the Riveter, The Problem We All Live With, Saying Grace, and the Four Freedoms series.

Question: what was the theme of most of his paintings?
Self-contained rewrite: What was the theme of most of Norman Rockwell's paintings?