ForeFlight does not have a night mode, but a good work around is to go to the iPad SETTINGS APP and tap GENERAL > ACCESSIBILITY > ACCESSIBILITY SHORTCUT > and select INVERT COLORS.
Image 1. GENERAL > ACCESSIBILITY
Image 2. ACCESSIBILITY SHORTCUT
Image 3. Select INVERT COLORS
Now, when you're in ForeFlight Mobile and want to invert the colors, simply press the iPad/iPhone home button 3 times in a row.