example.xaml

<Style TargetType="Entry">
    <Setter Property="TextColor" Value="{AppThemeBinding Light={StaticResource Black}, Dark={StaticResource White}}" />
    <Setter Property="FontFamily" Value="OpenSansRegular"/>
    <Setter Property="FontSize" Value="14" />
    <Setter Property="PlaceholderColor" Value="{AppThemeBinding Light={StaticResource LightGray}, Dark={StaticResource DarkGray}}" />
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup x:Name="CommonStates">
                <VisualState x:Name="Normal">
                    <VisualState.Setters>
                        <Setter Property="TextColor" Value="{AppThemeBinding Light={StaticResource Black}, Dark={StaticResource White}}" />
                    </VisualState.Setters>
                </VisualState>
                <VisualState x:Name="Disabled">
                    <VisualState.Setters>
                        <Setter Property="TextColor" Value="{AppThemeBinding Light={StaticResource LightGray}, Dark={StaticResource DarkGray}}" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>