Custom ThemeData Flutter

build.dart

@override
Widget build(BuildContext context) {
  final Brightness platformBrightness = Theme.of(context).brightness;
  final bool darkTheme = platformBrightness == Brightness.dark;

  return CustomAppTheme(
    customAppTheme:
        darkTheme ? CustomAppThemeData.dark : CustomAppThemeData.light,
    child: Icon(Icons.add, color: CustomAppTheme.of(context).addColor,),
  );
}

CustomAppTheme.dart

import 'package:calendarflutter/style/custom_app_theme_data.dart';
import 'package:flutter/material.dart';

class CustomAppTheme extends InheritedWidget {
  CustomAppTheme({
    Key key,
    @required Widget child,
    this.customAppTheme,
  }) : super(key: key, child: child);

  final CustomAppThemeData customAppTheme;

  static CustomAppThemeData of(BuildContext context) {
    return context
        .dependOnInheritedWidgetOfExactType<CustomAppTheme>()
        .customAppTheme;
  }

  @override
  bool updateShouldNotify(CustomAppTheme oldWidget) =>
      customAppTheme != oldWidget.customAppTheme;
}

CustomAppThemeData.dart

import 'package:flutter/material.dart';

class CustomAppThemeData {
  final Color plusColor;

  const CustomAppThemeData({
    @required this.plusColor,
  });

  static CustomAppThemeData get dark {
    return CustomAppThemeData(
      plusColor: Colors.red,
    );
  }

  static CustomAppThemeData get light {
    return CustomAppThemeData(
      plusColor: Colors.green,
    );
  }
}