Shadows are not defined

[WrongBit]

I referenced standard library:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.8\PresentationFramework.Aero2.dll

And for DarkTheme.xaml VS complains:

Error XDG0008 The name "SystemDropShadowChrome" does not exist in the namespace "clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero2"

(around 20 such messages) I don't know anything newer than FW 4.8 - where did you get your shadows??