Пример переключения темы в простом варианте

switch_theme.dart

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

void main() {
  runApp(const App());
}

class App extends StatefulWidget {
  const App({Key? key}) : super(key: key);

  @override
  State<App> createState() => _AppState();
}

class _AppState extends State<App> {
  @override
  Widget build(BuildContext context) {
    return ValueListenableBuilder<bool>(
      valueListenable: AppSettings.themeIsLight,
      builder: (_, isLight, __) => MaterialApp(
        title: 'Places',
        theme: isLight ? AppTheme.lightTheme : AppTheme.darkTheme,
        home: const SettingsScreen(),
      ),
    );
  }
}

/// Экран настроек
class SettingsScreen extends StatelessWidget {
  const SettingsScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final isDark = Theme.of(context).brightness == Brightness.dark;

    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: [
          ListTile(
            title: const Text('Тема темная'),
            trailing: CupertinoSwitch(
              value: isDark,
              onChanged: (value) {
                AppSettings.themeIsLight.value = !value;
              },
            ),
          ),
        ],
      ),
    );
  }
}

/// Хранилище настроек (временно, до прохождения стейт менеджеров)
abstract class AppSettings {
  /// настройки темы
  static ValueNotifier<bool> themeIsLight = ValueNotifier(true);
}

/// Тема приложения
class AppTheme {
  static ThemeData get lightTheme => _buildTheme();

  static ThemeData get darkTheme => _buildThemeDark();

  AppTheme._();

  /// Светлая тема
  // ignore: long-method
  static ThemeData _buildTheme() {
    final _base = ThemeData.light();

    return _base.copyWith(
      brightness: Brightness.light,
    );
  }

  /// Темная тема
  // ignore: long-method
  static ThemeData _buildThemeDark() {
    final _base = ThemeData.dark();

    return _base.copyWith(
      brightness: Brightness.dark,
    );
  }
}

https://dartpad.dev/?id=8d292cae6286002de74a78af58afc3fa