AquaToolbarButton.swift

struct AquaButtonStyle: ButtonStyle {
  func makeBody(configuration: Self.Configuration) -> some View {
    AquaButton()
  }
}

struct AquaButton: View {
  var body: some View {
    GeometryReader { geometry in
      let topHighlightHeight = geometry.size.height * 0.3
      let bottomHighlightOffset = geometry.size.height * 0.6
      ZStack {
        Capsule() //Behind highlight
            .foregroundColor(.white)
            .offset(y: 1.5)
        Group {
        //Main colour (foregroundColor of the Button)
        Capsule()
        //Gradient on top of the main colour
        Capsule() //Bottom highlight
          .foregroundColor(.white)
          .offset(y: bottomHighlightOffset)
          .blur(radius: 3)
        Group {
          Capsule() //Top highlight
            .frame(height: topHighlightHeight)
            .foregroundColor(Color.white)
            .offset(y: 1)
            .blur(radius: 2)
            .padding(.horizontal, 5)
        }
        .frame(maxHeight: .infinity, alignment: .top)
        
        //The outline for the button
        Capsule().stroke(Color.black, lineWidth: 3.5)
        }
        //Make sure none of the layers go outside the shape
        .clipShape(Capsule())
      }
    }
  }
}