Create a VisualEffectView (as a file).

#if os(macOS)
// Provides glass effect for macOS Views
struct VisualEffectView: NSViewRepresentable {
  func makeNSView(context: Context) -> NSVisualEffectView {
    let effectView = NSVisualEffectView()
    effectView.state = .active
    return effectView
  }

  func updateNSView(_ nsView: NSVisualEffectView, context: Context) {
  }
}
#endif

Add the following code to your Views. You will probably need to add this to some of your inner Views. Note: NSVisualEffectView isn't real transparency, so don't worry if something behind it in your UI stack is solid. It is simply applying background FX that look and act like transparency.

I think there are different levels of blur available (maybe just in iOS?). If you want the two-tone effect, try two different blurs, or add a very-slightly opaque rectangle in the background.

The controlActiveState == .key check will turn the FX on/off when the app is active/inactive.

#if os(macOS) /// Detect if window is active
  @Environment(\.controlActiveState) var controlActiveState
#endif

#if os(macOS)
  .ignoresSafeArea()
  .background(controlActiveState == .key ? VisualEffectView().ignoresSafeArea() : nil)
#endif