How to set text color theme for entire app in flutter

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);
const Color PRIMARY_COLOR = Colors.indigo;
const Color ACCENT_COLOR = Colors.indigoAccent;
const Color BACKGROUND_COLOR = Colors.grey;


   ThemeData _buildTheme() {       
         final ThemeData base = ThemeData.light();
        return base.copyWith(
          primaryColor: PRIMARY_COLOR,
          accentColor: ACCENT_COLOR,
          backgroundColor: BACKGROUND_COLOR,
          appBarTheme: _appBarTheme(base.appBarTheme),
          textTheme: _textTheme(base.textTheme),
          buttonTheme: base.buttonTheme.copyWith(
            buttonColor: PRIMARY_COLOR,
          ),
        );
      }

AppBarTheme _appBarTheme(AppBarTheme base) => base.copyWith(
      color: BACKGROUND_COLOR,
      brightness: Brightness.light,
      textTheme: TextTheme(
          headline1: TextStyle(
              fontWeight: FontWeight.w600,
              fontSize: 18.0,
              fontFamily: 'Montserrat',
              color: ACCENT_COLOR),),
      iconTheme: IconThemeData(color: ACCENT_COLOR));

TextTheme _textTheme(TextTheme base) {
    return base.copyWith(
      headline1: base.headline1.copyWith(
          fontWeight: FontWeight.w500,
          fontFamily: 'Montserrat',
          color: ACCENT_COLOR),
      subtitle1: base.subtitle1.copyWith(
          fontSize: 20,
          //    fontWeight: FontWeight.w600,
          fontFamily: 'Montserrat',
          color: ACCENT_COLOR),
      caption: base.caption.copyWith(
          fontWeight: FontWeight.w400,
          fontFamily: 'Montserrat',
          //  fontSize: TEXT_FONT_SIZE,
          color: ACCENT_COLOR),
      bodyText1: base.bodyText1.copyWith(
          fontWeight: FontWeight.w400,
          fontFamily: 'Montserrat',
          //   fontSize: TEXT_FONT_SIZE,
          color: ACCENT_COLOR),
      bodyText2: base.bodyText2.copyWith(
          fontWeight: FontWeight.w500,
          fontFamily: 'Montserrat',
          //  fontSize: TEXT_LARGE_FONT_SIZE,
          color: ACCENT_COLOR),
      button: TextStyle(
          color: Colors.white,
          fontWeight: FontWeight.w500,
          fontFamily: 'Montserrat',
          fontSize: 16),
    );
  }


void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: _buildTheme(),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text('Hello, World!', style: Theme.of(context).textTheme.headline1);
  }
}