Do pilots keep light in the cockpit during long night flights?
White lights are usually off in the cockpit in night flights to protect pilots' night vision. Red lights are sometimes used as these do not impact night vision.
One of the challenges you'll run into is knowing when it's night, as the start of night varies by latitude and the exact time. Phones generally set their time from the mobile network, which may be absent on long flights, especially those over water. A pilot flying a jet over the pacific may may have a phone that is hours off the local time.
Putting my (albeit old and tattered) mobile developer hat on I would suggest you look for ways the phone can tell you whether it thinks it is night or day and use that, plus a manual selection for day/night mode. Look at the Android UIModeManager as a way this could be done native.