Install the Xamarin.Forms.GoogleMaps
Nuget Package (source code available on GitHub)
which has already implemented it on Xamarin.Forms.
You can refer to the MapStylePage sample available here which basically explains you how to create original map styles using MapStyle With Google. You can use the wizard, select the Night theme from there and get the corresponding json style, which you’ll use on your Xamarin app.
solved How set auto day/night mode google maps in Xamarin Forms