I really like the new AppThemeBinding markup extension in Xamarin.Forms to easily handle Dark and Light modes on the mobile device. It can be used in combination with the StaticResource markup extension.

Below is an example:

BackgroundColor="{AppThemeBinding
        Light={StaticResource LightBackgroundColor},
        Dark={StaticResource DarkBackgroundColor}}"
<ContentPage.Resources>
    <Color x:Key="LightBackgroundColor">WhiteSmoke</Color>
    <Color x:Key="DarkBackgroundColor">Black</Color>
</ContentPage.Resources>

screen recording on iPhone Simulator