At Evolve it was announced that Xamarin were going to support theming for Xamarin forms, there is an active discussion here.
I did ask the question about custom controls and this was the response:
"The ability to theme third party controls comes from 1 of 2 mechanisms. First if the third party control is composed of smaller base controls and uses Control or Item templating it will just work automatically. If however if is not, the vendor would need to release its own theming for the control at this time."
With that in mind, do Telerik intend to support this feature which we intend to use in the future, I know it's early days but I thought I'd highlight this new features in Forms.
Also, will the Telerik controls work with the designer preview announced at Evolve?again early days and I think it's only in the Alpha channel for Xamarin Studio but Visual Studio support is coming.