Template Binding with Attached Properties
I think TemplateBinding is evaluated at compile time so you can't dynamically set a TemplateBinding in your Setter, try using Binding instead (see below).
<Setter TargetName="border" Property="BorderBrush" Value="{Binding Path=(local:ThemeProperties.ButtonBorderColour), RelativeSource={RelativeSource TemplatedParent}}"/>Hope this helps.
Try this:
<Setter TargetName="border" Property="BorderBrush" Value="{TemplateBinding Path=(local:ThemeProperties.ButtonBorderColour)}" />The difference being that parentheses around the property indicate that it is an attached property.