apply_theme.dart

final kBaseThemeData = ThemeData(
    primaryColor: Color.fromRGBO(58, 66, 86, 1.0),
    backgroundColor: Color.fromRGBO(58, 66, 86, 1.0)
);

class _MyAppState extends State<MyApp> {
  int _currentIndex = 0;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: kBaseThemeData,
      home: Scaffold(...)
      ...
    );
  }
}