如何在flutter中反转TextFormField的颜色？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final theme = ThemeData(
      primaryColor: Colors.blue, // 设置主题色
      textTheme: TextTheme(
        bodyText2: TextStyle(color: Colors.black), // 设置文字样式
      ),
      inputDecorationTheme: InputDecorationTheme(
        filled: true,
        fillColor: Theme.of(context).textTheme.bodyText2!.color!.withOpacity(0.5), // 设置输入框填充色
      ),
    );

    return MaterialApp(
      theme: theme, // 应用主题样式
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo'),
        ),
        body: Center(
          child: Padding(
            padding: const EdgeInsets.all(20.0),
            child: TextFormField(
              decoration: InputDecoration(
                labelText: 'Username',
              ),
            ),
          ),
        ),
      ),
    );
  }
}