TemplateBinding from a Style DataTrigger In ControlTemplate - wpf

Question

In the following XAML I'm using a Rectangle with a Border as the Template for a ToggleButton.
I want the BorderBrush to be a different colour to reflect the changing value of ToggleButton.IsChecked.
Unfortunately my attempt here of using a TemplateBinding in the DataTrigger doesn't work. What do I need to do instead?
<StackPanel Orientation="Horizontal">
<StackPanel.Resources>
<ControlTemplate x:Key="ButtonAsSwatchTemplate">
<Border BorderThickness="1">
<Border.Style>
<Style>
<Setter Property="BorderBrush" Value="Gainsboro" />
<Style.Triggers>
<!-- TemplateBinding doesn't work.-->
<DataTrigger
Binding={TemplateBinding Property=IsChecked}
Value="True">
<Setter Property="BorderBrush" Value="Black" />
</DataTrigger>
</Style.Triggers>
</Style>
</Border.Style>
<Rectangle Fill="{TemplateBinding Property=Background}"
Width="15" Height="15" />
</Border>
</ControlTemplate>
</StackPanel.Resources>
<ToggleButton Template="{StaticResource ButtonAsSwatchTemplate}"
HorizontalAlignment="Center" VerticalAlignment="Center"
Margin="2" Background="Red" />
</StackPanel>
EDIT
When I build and reload the designer I get the following error:
Error 1 Property 'Binding' does not support values of type 'TemplateBindingExpression'.
SOLUTION
<StackPanel Orientation="Horizontal">
<StackPanel.Resources>
<ControlTemplate x:Key="ButtonAsSwatchTemplate">
<Border x:Name="SwatchBorder" BorderThickness="1">
<Rectangle Fill="{TemplateBinding Property=Background}" Width="15" Height="15" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="ToggleButton.IsChecked" Value="True">
<Setter TargetName="SwatchBorder" Property="BorderBrush" Value="Yellow" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</StackPanel.Resources>
<RadioButton Template="{StaticResource ButtonAsSwatchTemplate}"
GroupName="CropGuidesColourRadioButtonGroup"
IsChecked="{Binding Checked}" Margin="2" Background="Red" />
<RadioButton Template="{StaticResource ButtonAsSwatchTemplate}"
GroupName="CropGuidesColourRadioButtonGroup"
IsChecked="{Binding Checked}" Margin="2" Background="Black" />
...
</StackPanel>

Answer 1

You could use a Trigger in the ControlTemplate, e.g.
<StackPanel Orientation="Horizontal">
<StackPanel.Resources>
<ControlTemplate x:Key="ButtonAsSwatchTemplate">
<Border x:Name="myBorder" BorderBrush="Gainsboro" BorderThickness="1">
<Rectangle Fill="{TemplateBinding Property=Background}" Width="15" Height="15" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="ToggleButton.IsChecked" Value="True">
<Setter TargetName="myBorder" Property="BorderBrush" Value="Black" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</StackPanel.Resources>
<ToggleButton Template="{StaticResource ButtonAsSwatchTemplate}"
HorizontalAlignment="Center" VerticalAlignment="Center"
Margin="2" Background="Red" />
</StackPanel>

Answer 2

One problem I see right away is that you are setting BorderBrush as a local property value on Border. This will always override the value applied by the style. Try removing the explicit BorderBrush attribute and see if that works.
Border BorderBrush="Gainsboro" BorderThickness="1"
Dependency Property Value Inheritance