Problem is, PickerFlyout does not appear to be "styleable", and in fact is simply a Popup control. Using XamlSpy we can see that the child of the Popup is a PickerFlyoutPresenter, which can be styled. But if you attempt something like:
<Style TargetType="PickerFlyoutPresenter"> <Setter Property="Background" Value="White" /> </Style>
on a page or control level you will find that it has no effect on your Flyout. This is because the Flyout is not a child of the page or control. Simply move it to your App.xaml to have this style applied throughout your app.