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.