a class method named ._() function in dart?

I've seen this code can anyone please explain to me what the AppTheme._() means, as I've read about its singleton class in dart but I really can't understand how it works.

class AppTheme {
  AppTheme._();

  static const Color notWhite = Color(0xFFEDF0F2);
  static const Color nearlyWhite = Color(0xFFFEFEFE);
  static const Color white = Color(0xFFFFFFFF);
  static const Color nearlyBlack = Color(0xFF213333);

  ...
}

AppTheme._(); is a named constructor (another examples might be the copy constructor on some objects in the Flutter framework: ThemeData.copy(...);).

In dart, if the leading character is an underscore, then the function/constructor is private to the library. That's also the case here, and the underscore is also the only character, so I'd imagine whoever wrote this constructor didn't plan for that constructor to ever be called at all.

The AppTheme._(); isn't necessary unless you don't want AppTheme to ever be accidentally instantiated using the implicit default constructor.

It is to make the class non-instantiable.

More on https://www.woolha.com/tutorials/dart-prevent-instantiation-of-class#:~:text=Creating%20Private%20Constructor%20to%20Prevent,(underscore)%20which%20means%20private.

Also, I think this summarise why we need it in the first place "If a class has one or more private constructors and no public constructors, other classes (except nested classes) cannot create instances of this class. " from https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/private-constructors