One possible theme that may be used in an analysis from the historical context could be that of wealth. How did people of the time use their money? New York was filled with wealthy families, and the rate of immigration would have supplied an abundance of servants. Transportation was booming, people no longer necessarily took cruises, but rather indulged in large toys (yachts), and don't forget the freedom found in cars. Across the country and across the world, people were moving about more and more.
Lily was a social butterfly. She moved from event to event and boat to boat. Families in the novel parallel well known families of the time (do some research). Society was changing, and old money made room for new money to join their ranks. Families also went bankrupt, fortunes were lost, and lives changed. Have fun with this. It's a great topic!