You can find documentation for creating custom renderers under the "How To" sections of controls that have tutorials for renderers.
For example, there are two for the DataForm control:
Note that those tutorials are geared towards customizing the native editors to provide a better experience for a specific data type. Each platform's native editor will still be a different control and has it's default appearance (e.g. text entry controls are different).
All of the controls have a renderer that you can inherit from, even if you don't see a documentation topic for it. You can find the renderer class name in the following documentation articles:
Further Support for Custom UI
As far as creating custom styling so that the controls match on each platform, as I stated before this goes against the design of UI for Xamarin and you'll likely find it a difficult experience to completely override all the components.
Creating custom UI styling with the controls falls outside the scope
of UI for Xamarin support, however you can reach out to our Professional Services partners who can work with you to create a solution. Go here
to contact Professional Services.
I hope I was able to answer your question to your satisfaction.
Lance | Tech Support Engineer, Sr.
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