Set border color of editable ComboBox on focus

Question

What I want to do, is set BorderBrush of editable ComboBox. I'm using slightly modified default WPF templates. They are structured like below:
<ControlTemplate x:Key="ComboBoxEditableTemplate" TargetType="{x:Type ComboBox}">
<Grid x:Name="templateRoot" ...>
<ToggleButton x:Name="toggleButton" ... />
<Border x:Name="border" ...>
<TextBox x:Name="PART_EditableTextBox" ... />
<!-- textbox with IsFocused property which should be focus trigger -->
</Border>
</Grid>
</ControlTemplate>
ToggleButton template:
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border x:Name="templateRoot" ...> <!-- first border brush I want to set on focus -->
<Border x:Name="innerBorder" ...> <!-- second border brush I want to set on focus -->
<Border x:Name="splitBorder" ...>
<Path x:Name="arrow" ... />
</Border>
</Border>
</Border>
</ControlTemplate>
And now, what should happen.
When PART_EditableTextBox.IsFocused is equal to true then set
templateRoot.BorderBrush and innerBorder.BorderBrush to another color (for example red and blue).
It would be very simple if there was only one BorderBrush to set, because I could use
TemplateBinding to bind this property to ToggleButton element.
For me, the problem are nested templates. I don't know how to refer to inside of another template.
I hope you can help me.
It would be very good if I didn't need to use C# code, just XAML.

Answer 1

>>I don't know how to refer to inside of another template.
You cannot. But since the PART_EditableTextBox is part of the ControlTemplate of the ComboBox you could add the trigger to this control template and then for example set the BorderBrush property and the Background propery of the ToggleButton and bind these
to the BorderBrush properties of the Borders in its ControlTemplate:
<ControlTemplate x:Key="ComboBoxEditableTemplate" TargetType="{x:Type ComboBox}">
<TextBox x:Name="PART_EditableTextBox" ... />
<ToggleButton x:Name="toggleButton" ... />
<ControlTemplate.Triggers>
<Trigger Property="IsFocused" Value="True" SourceName="PART_EditableTextBox">
<Setter Property="BorderBrush" TargetName="toggleButton" Value="Red"/>
<Setter Property="Background" TargetName="toggleButton" Value="Blue" />
</Trigger>
</ControlTemplate.Triggers>
<Style x:Key="ComboBoxToggleButton" TargetType="{x:Type ToggleButton}">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Setter Property="IsTabStop" Value="false"/>
<Setter Property="Focusable" Value="false"/>
<Setter Property="ClickMode" Value="Press"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border x:Name="templateRoot" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="2">
<Border x:Name="innerBorder" BorderBrush="{TemplateBinding Background}" BorderThickness="2">
<Border x:Name="splitBorder">
<Path x:Name="Arrow" Data="{StaticResource DownArrowGeometry}"
Fill="Black" HorizontalAlignment="Center" Margin="0,1,0,0" VerticalAlignment="Center"/>
</Border>
</Border>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
If you don't want to use the Background property for this, the other option is to create your own custom class that derives from the ToggleButton class and add a dependency property called "InnerBackground" or something to this class and replace
the ToggleButton with your custom ToggleButton and replace the binding to the Background property with a binding to the new dependency property and set this one in the control template for the ComboBox.
Hope that helps.
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