Apply theme from resources

[Mehrdad]

I've added a resource file to my WPF application and in it I have:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation">

<Style TargetType="telerik:RadGridView" BasedOn="{StaticResource {telerik:ThemeResourceKey ThemeType=telerik:Office_BlueTheme, ElementType=telerik:RadGridView}}" />
</ResourceDictionary>

and in App.xaml I have:
<Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Dictionary1.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>


And in a usercontrol I've added a RadGridView; Now the group section and gridline borders are blue but the header remains black.
why the theme dosen't apply to column header?