change RadNumericUPDown background by style

Question

hello i'm trying to add style which change radnumericupdown background when it got the focus but this doesn't work 

<Style TargetType="{x:Type telerik:RadNumericUpDown}" >

               

        <Style.Triggers>

            <Trigger Property="IsFocused" Value="true">

                <Setter Property="Background" Value="#FEFCD7" />

            </Trigger>

            <Trigger Property="IsFocused" Value="false">

                <Setter Property="Background" Value="White"/>

            </Trigger>

        </Style.Triggers>

    </Style>