Previewer background colour or style setting?

Hi, when I use the Xamarin Forms Previewer in Visual Studio 2017 (15.6.7) my app is shown in 'dark' mode or style i.e. has a black background, text is white (or light gray) yet when I run the app on my phone again using VS the background is white, text dark.

Is there some setting I need to change to make the previewer the same as my phone?

Note this is the Android preview.

Thanks John