Demo

textfield.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        title: 'Flutter Demo',
        home: new Scaffold( // 1
            appBar: new AppBar(
                title: new Text("Demo"), // screen title
            ),
            body: new Container(
                color: Colors.blueGrey,
                child: myWidget()
            )
        )
    );
  }

Widget myWidget() {
            var textStyle = new TextStyle(
                        color:Colors.red,
                        fontSize: 14.0
                    );
    return new Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
            new Container(
                color: Colors.red,
                height: 100.0,
                width: 100.0,
            ),
            new TextField(
                keyboardType:TextInputType.number,
                textAlign: TextAlign.center,
                onChanged: (text) {
                },
                decoration: new InputDecoration(
                    hintText: 'placeholder',
                    fillColor: Colors.yellow,
                    filled: true,
                    hintStyle: textStyle
                ),
                style: textStyle
            ),
            new Container(
                color: Colors.green,
                height: 100.0,
                width: 100.0,
            )
        ],
    );
}