TemplateBinding to SolidColorBrush won't work when overriding Button ControlTemplate

Hi, I want to create a custom control which will extend the build-in Button and allow user to set the pointer-over Background color. I have added a dependency property of type Brush, and "properly" set the binding in the according visual state of ControlTemplate using TemplateBinding. But it still won't work - when I move mouse over the button, nothing happens.

Here is code:

[Generic.xaml](I've bolded the font of all the related code. )

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="using:App38">

    <Style TargetType="local:ButtonEx">
        <Setter Property="Background"
                Value="{ThemeResource ButtonBackgroundThemeBrush}" />
        <Setter Property="Foreground"
                Value="{ThemeResource ButtonForegroundThemeBrush}" />
        <Setter Property="BorderBrush"
                Value="{ThemeResource ButtonBorderThemeBrush}" />
        <Setter Property="BorderThickness"
                Value="{ThemeResource ButtonBorderThemeThickness}" />
        <Setter Property="Padding"
                Value="12,4,12,4" />
        <Setter Property="HorizontalAlignment"
                Value="Left" />
        <Setter Property="VerticalAlignment"
                Value="Center" />
        <Setter Property="FontFamily"
                Value="{ThemeResource ContentControlThemeFontFamily}" />
        <Setter Property="FontWeight"
                Value="SemiBold" />
        <Setter Property="FontSize"
                Value="{ThemeResource ControlContentThemeFontSize}" />

        <Setter Property="PointerOverBackground"
                Value="{ThemeResource ButtonPointerOverBackgroundThemeBrush}" />
        
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:ButtonEx">
                    <Grid>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="Normal" />
                                <VisualState x:Name="PointerOver">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background"
                                                                       Storyboard.TargetName="Border">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{TemplateBinding PointerOverBackground}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="ContentPresenter">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonPointerOverForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Pressed">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background"
                                                                       Storyboard.TargetName="Border">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonPressedBackgroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="ContentPresenter">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonPressedForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Disabled">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background"
                                                                       Storyboard.TargetName="Border">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonDisabledBackgroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush"
                                                                       Storyboard.TargetName="Border">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonDisabledBorderThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
                                                                       Storyboard.TargetName="ContentPresenter">
                                            <DiscreteObjectKeyFrame KeyTime="0"
                                                                    Value="{ThemeResource ButtonDisabledForegroundThemeBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                            <VisualStateGroup x:Name="FocusStates">
                                <VisualState x:Name="Focused"/>
                                <VisualState x:Name="Unfocused" />
                                <VisualState x:Name="PointerFocused" />
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Border x:Name="Border"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="{TemplateBinding BorderThickness}"
                                Background="{TemplateBinding Background}"
                                Margin="3">
                            <ContentPresenter x:Name="ContentPresenter"
                                              AutomationProperties.AccessibilityView="Raw"
                                              ContentTemplate="{TemplateBinding ContentTemplate}"
                                              ContentTransitions="{TemplateBinding ContentTransitions}"
                                              Content="{TemplateBinding Content}"
                                              HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                              Margin="{TemplateBinding Padding}"
                                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
                        </Border>
                        
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

[ButtonEx.cs]

using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;


namespace App38
{
    public sealed class ButtonEx : Button
    {
        public SolidColorBrush PointerOverBackground
        {
            get { return (SolidColorBrush)GetValue(PointerOverBackgroundProperty); }
            set { SetValue(PointerOverBackgroundProperty, value); }
        }
        public static readonly DependencyProperty PointerOverBackgroundProperty =
            DependencyProperty.Register("PointerOverBackground", typeof(SolidColorBrush), typeof(ButtonEx), new PropertyMetadata(null));


        public ButtonEx()
        {
            this.DefaultStyleKey = typeof(ButtonEx);
            
        }
    }
}