How to include app theme in a multibinding?

[Ghostbird]

I have a view that renders a non-bindable content based on a viewmodel value. Therefore I need to bind on that value, and the theme. This will allow me to rerender the content when the theme changes.
However I can't figure out how to do this. I wrote the logical:

new MultiBinding
{
  Binding = [
    new Binding(nameof(ViewModel.Value)),
    new AppThemeBinding()
  ]
}

However this doesn't compile as ,for reasons unclear to me, the AppThemeBinding is internal, which seems to make no sense. Why am I not allowed to use it? In these kind of use cases it seems that it is the correct and sensible binding to use.

How can I still make a binding that will recalculate the resulting value when the app theme changes?

[Ghostbird]

I‌ had to build a workaround:

• I reworked the bound view-model value to contain a model that incorporated theme specific data.
• I added an extension method Themed to that model to set or override that themed data in that model.
• I changed the view-model to be disposable and added:

    // At the end of the constructor
    Value = /* Value construction code */.Themed(Application.Current!.RequestedTheme);
    Application.Current!.RequestedThemeChanged += OnThemeChanged;
  }
  
  void OnThemeChanged(object? sender, AppThemeChangedEventArgs e)
  => Value = Value.Themed(e.RequestedTheme);
  
  public void Dispose()
  {
    GC.SuppressFinalize(this);
    Application.Current!.RequestedThemeChanged -= OnThemeChanged;
  }

[Ghostbird]

Still, I think it's ridiculous that I had to invent all that, simply because the AppThemeBinding is marked internal for no apparent reason.