Answer by Liyun Zhang - MSFT for Auto applying element styles

You can use ApplyToDerivedTypes = "True" in the Styles.xaml to do that:

<Style x:Key="ContentPageStyle" TargetType="ContentPage" ApplyToDerivedTypes = "True"> <Setter Property="BackgroundColor" Value="{AppThemeBinding Light={StaticResource White}, Dark={StaticResource MyColor}}" /></Style>

In addition, don't forget to add the color in the Colors.xaml:

<Color x:Key="MyColor">#020202</Color>

And you will find <Style TargetType="Page" ApplyToDerivedTypes = "True"> in the Styles.xaml. You can just change the setter in it and the contentpage will auto apply it.