WPF Trigger Binding is not working - wpf

Question

I've read a couple of posts that handles the same issue, but I don't find a solution.
What I want to achieve is, that the WPF checkbox changes its background and borderbrush colors on mouse hover. The colors shouldn't be hardcoded in XAML. Background and borderbrush properties should be bound to properties in a POCO class.
This is, what I've done:
first, I created a POCO class, that derives from WPF CheckBox. I created a dependency property called MouseOverBorderBrush:
Public Class efCheckBox
Inherits CheckBox
Implements INotifyPropertyChanged
Public Shared ReadOnly MouseOverBorderBrushProperty As DependencyProperty =
DependencyProperty.Register("MouseOverBorderBrush",
GetType(Brush), GetType(efCheckBox))
Public Property MouseOverBorderBrush As Brush
Get
Return CType(GetValue(MouseOverBorderBrushProperty), Brush)
End Get
Set(ByVal value As Brush)
PropertyHasChanged(MouseOverBorderBrushProperty, value)
End Set
End Property
Private Sub PropertyHasChanged(dp As DependencyProperty, value As Object, <System.Runtime.CompilerServices.CallerMemberName> Optional p As String = Nothing)
SetValue(dp, value)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(p))
End Sub
Second, I created a ResourceDictionary called "efComboBox". I did this by cloning the template of the original WPF CheckBox, so the following code is - with two exceptions - the original CheckBox definition:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:emmbeeFramework"
xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero">
<SolidColorBrush x:Key="CheckBoxFillNormal" Color="#F4F4F4"/>
<SolidColorBrush x:Key="CheckBoxStroke" Color="#8E8F8F"/>
<Style x:Key="EmptyCheckBoxFocusVisual" TargetType="{x:Type local:efCheckBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="1" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="CheckRadioFocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Rectangle Margin="14,0,0,0" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="efCheckBoxStyle" TargetType="{x:Type local:efCheckBox}">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="Background" Value="{StaticResource CheckBoxFillNormal}"/>
<Setter Property="BorderBrush" Value="{StaticResource CheckBoxStroke}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="FocusVisualStyle" Value="{StaticResource EmptyCheckBoxFocusVisual}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:efCheckBox}">
<BulletDecorator x:Name="bullet" Background="Transparent" SnapsToDevicePixels="true">
<BulletDecorator.Bullet>
<Themes:BulletChrome BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" IsChecked="{TemplateBinding IsChecked}" RenderPressed="{TemplateBinding IsPressed}"/>
</BulletDecorator.Bullet>
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</BulletDecorator>
<ControlTemplate.Triggers>
<Trigger Property="HasContent" Value="true">
<Setter Property="FocusVisualStyle" Value="{StaticResource CheckRadioFocusVisual}"/>
<Setter Property="Padding" Value="4,0,0,0"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" Value="{Binding local:efCheckBox.MouseOverBorderBrush}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
The two exceptions:
I've chanted the "TargetType" from "CheckBox" to"efCheckBox"
I've added a trigger "IsMouseOver" near the end of the XAML code
This binding is not working:
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" Value="{Binding local:efCheckBox.MouseOverBorderBrush}"/>
</Trigger>
I also tried, but it doesn't work:
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" Value="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:efCheckBox}}, Path=MouseOverBorderBrush}"/>
If I hardcode the brush, it works:
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" Value="Red"/>
</Trigger>
<Setter Property="BorderBrush" Value="Red">
Could someone please help me?