Customizing standard controls

Is there a way to customize a little standard controls looking in a cross-platform way?
Like, I need to disable border around an Entry. Is it possible?
Or, what if I want to draw only one border (let's say at the bottom) for Entry or Frame. Should I make own control with own renderer for this purpose or I can customize this somehow?

Also, I didn't understand how to make own controls with custom rendering? Is there a way to do some simple drawing using Xamarin.Forms or I should make own renderer per each platform? How? I didn't see any sample of custom renderer implementation.

And last, can somebody point me to the place in documentation where CSS styling is described?

Thanks.