Styling numeric boxes on focus

[vaclav-domin]

How styling numeric boxec on got focus?
style like this not working

<Style TargetType="numbers:IntBox">
         <Style.Triggers>
            <Trigger Property="IsFocused"
                      Value="True">
                  <Setter Property="Background"
                           Value="Yellow" />
                </Trigger>
</Style>