Final answer:
Without explicit context, the specific American custom the narrator's mother has adopted is unknown, but common American customs include wearing jeans and consuming American products and entertainment.
Step-by-step explanation:
The American custom that the narrator's mother has adopted to fit in for her part-time job is not specified in the context provided. Without explicit context, the specific American custom the narrator's mother has adopted is unknown, but common American customs include wearing jeans and consuming American products and entertainment.
However, referencing global influences, one can infer that American customs and looks that have been widely adopted around the world after World War II, include wearing Levi's jeans, consuming products like Coca-Cola, and engaging with American entertainment such as Hollywood movies and music.