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