maui-linux/Xamarin.Forms.Platform.WPF/Themes/FormsContentPage.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:controls="clr-namespace:Xamarin.Forms.Platform.WPF.Controls">

    <Style TargetType="controls:FormsContentPage">
        <Setter Property="Background" Value="White"/>
        <Setter Property="HorizontalAlignment" Value="Stretch"/>
        <Setter Property="VerticalAlignment" Value="Stretch"/>
        <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
        <Setter Property="TitleBarBackgroundColor" Value="{DynamicResource DefaultTitleBarBackgroundColor}"/>
        <Setter Property="TitleBarTextColor" Value="{DynamicResource DefaultTitleBarTextColor}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="controls:FormsContentPage">
                    <Border HorizontalAlignment="{TemplateBinding HorizontalAlignment}"
                            VerticalAlignment="{TemplateBinding VerticalAlignment}"
                            Background="{TemplateBinding Background}">
                        <ContentPresenter/>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>