A rounded ElevatedButton.

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: ElevatedButton(
            style: ElevatedButton.styleFrom(
              shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(16),
              ),
            ),
            onPressed: () => print('Pressed!'),
            child: Text('Press Me'),
          ),
        ),
      ),
    );
  }
}