Visual Customization of Controls

I am starting with Xamarin.Forms and I am wondering what is the best way to draw Custom Controls.

One example would be a Button that has a background gradient.
Another example is an Entry with a white background and a black border around it.

I am talking about customizing how they look (not how they work)

The question is, should I use code on each platform (android and ios) to customize the look of the controls, or there is a way to write once and customize for both platforms?

Thanks