The answer is flickering streetlights or light through a window. Film noir is based on mood of pessimism, fatalism, and menace. It is also originally applied on American thriller films and detective films. So, flickering street lights or light through a window explains that the best.
By what i have read about 80% of revenue comes from non-musicals
A ghost light is a light that is left energized on the stage of a theater when the theater is empty and would otherwise be dark