create static method
Implementation
static Future<ThemeManager> create({
required List<AppTheme> themes,
required String defaultThemeName,
}) async {
final defaultTheme = themes.firstWhere(
(t) => t.name == defaultThemeName,
orElse: () => themes.first,
);
final manager = ThemeManager._(themes: themes, defaultTheme: defaultTheme);
await manager._init();
return manager;
}