Simple Icon Button with Favorite icon.

icon_button.dart

import 'package:flutter/material.dart';

const 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: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  bool isFavorite = false;
  @override
  Widget build(BuildContext context) {
    return IconButton(
      icon: Icon(
        isFavorite ? Icons.favorite : Icons.favorite_outline,
        color: isFavorite ? Colors.red : Colors.grey,
      ),
      onPressed: () => setState(() => isFavorite = !isFavorite),
    );
  }
}