Flutter Material 3 switch sample for a StackOverflow question https://stackoverflow.com/questions/75526521/how-to-customize-flutter-material-3-switch.

flutter_switch.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData.dark(
        useMaterial3: true,
      ).copyWith(
        scaffoldBackgroundColor: Colors.black,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _value = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Switch(
          focusColor: Colors.blue,
          trackColor: MaterialStateProperty.resolveWith(
            (Set<MaterialState> states) {
              if (states.contains(MaterialState.selected)) {
                return Colors.red;
              }
              return Colors.red.withOpacity(.1);
            },
          ),
          overlayColor: MaterialStateProperty.resolveWith(
            (Set<MaterialState> states) {
              if (states.contains(MaterialState.disabled)) {
                return Colors.red;
              }
              return Colors.white.withOpacity(.1);
            },
          ),
          hoverColor: Colors.green,
          thumbColor: MaterialStateProperty.resolveWith(
            (Set<MaterialState> states) {
              if (states.contains(MaterialState.selected)) {
                return HSLColor.fromColor(Colors.red)
                    .withLightness(0.2)
                    .toColor();
              }
              return Colors.red;
            },
          ),
          splashRadius: 24,
          inactiveTrackColor: Colors.pink,
          activeTrackColor: Colors.green,
          thumbIcon: MaterialStateProperty.resolveWith(
            (Set<MaterialState> states) {
              if (states.contains(MaterialState.selected)) {
                return const Icon(Icons.check, color: Colors.red);
              }
              return null;
            },
          ),
          activeColor: Colors.lime,
          inactiveThumbColor: Colors.orange,
          value: _value,
          onChanged: (bool value) => setState(() => _value = !_value),
        ),
      ),
    );
  }
}