CustomButtonStyle.swift

import SwiftUI

enum StylesSet {
    struct CustomButton: PrimitiveButtonStyle {
        func makeBody(configuration: Configuration) -> some View {
                         Button(configuration)
                            .padding()
                            .frame(minWidth: 0, maxWidth: .infinity)
                            .background(LinearGradient(gradient: Gradient(colors: [Color.red, Color.green]), startPoint: .leading, endPoint: .trailing))
                            .cornerRadius(40)
        }
    }
}

struct CustomViews: View {

    var body: some View {
        NavigationView {
            List {
                Button(action: {
                    
                }, label: {
                    Text("Demo Button").foregroundColor(.white)
                }).buttonStyle(StylesSet.CustomButton())
            }
            .navigationTitle("Styling Demo")
        }
    }
}

struct CustomViews_Previews: PreviewProvider {
    static var previews: some View {
        CustomViews()
    }
}