Flutter example setting styles for all three button types in the top level ThemeData

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Button Styling Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        textButtonTheme: TextButtonThemeData(
          style: TextButton.styleFrom(
            foregroundColor: Colors.green,
            textStyle: TextStyle(fontWeight: FontWeight.bold),
          ),
        ),
        outlinedButtonTheme: OutlinedButtonThemeData(
          style: OutlinedButton.styleFrom(
            foregroundColor: Colors.orange,
            side: BorderSide(color: Colors.orange, width: 2),
            shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(8)),
          ),
        ),
        elevatedButtonTheme: ElevatedButtonThemeData(
          style: ElevatedButton.styleFrom(
            backgroundColor: Colors.blue,
            foregroundColor: Colors.white,
            shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
            elevation: 5,
            padding: EdgeInsets.symmetric(horizontal: 20, vertical: 10),
          ),
        ),
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Button Styles')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextButton(onPressed: () {}, child: Text('Text Button')),
            SizedBox(height: 20),
            OutlinedButton(onPressed: () {}, child: Text('Outlined Button')),
            SizedBox(height: 20),
            ElevatedButton(onPressed: () {}, child: Text('Elevated Button')),
          ],
        ),
      ),
    );
  }
}