themeMode property Null safety
If set to ThemeMode.system, the choice of which theme to use will be based on the user's system preferences. If the MediaQuery.platformBrightnessOf is Brightness.light, theme will be used. If it is Brightness.dark, darkTheme will be used (unless it is null, in which case theme will be used.
The default value is ThemeMode.system.
final ThemeMode? themeMode;