How to set Flutter app theme as to dark by default?

You need to use ThemeMode

• Describes which theme will be used by MaterialApp.

SAMPLE CODE

themeMode: ThemeMode.dark,//Always use the dark mode (if available) regardless of system preference.


themeMode: ThemeMode.light,//Always use the light mode regardless of system preference.


themeMode: ThemeMode.system,//Use either the light or dark theme based on what the user has selected in the system settings.


themeMode: ThemeMode.values,//A constant List of the values in this enum, in order of their declaration.

How To use ThemeMode in MaterialApp

MaterialApp(
      debugShowCheckedModeBanner: false,
      theme:
          ThemeData(primarySwatch: Colors.blue, brightness: Brightness.light),
      themeMode: ThemeMode.dark,
      darkTheme: ThemeData(brightness: Brightness.dark),
      home: SafeArea(
          child:Scaffold(

          ) ),
    );

The recommended method is to use ColorScheme.

var mode = ThemeMode.light; // or ThemeMode.dark

MaterialApp(
  theme: ThemeData.from(colorScheme: ColorScheme.light()),
  darkTheme: ThemeData.from(colorScheme: ColorScheme.dark()),
  themeMode: mode,
  home: //...
)