How to override brushes

Hi,

I spent a couple of hours trying to override some brushes, but with no success...

Here is an example (see the part in bold). What I get is a compilation error: Type 'themes:AssetResourceKeys' was not found.

What's the correct definition?

Thanks,
Sam

<Window.Resources>

<Style TargetType="{x:Type docking:AutoHideHost}"><Setter Property="Padding" Value="0" /></Style>

<LinearGradientBrush x:Key="{x:Static themes:AssetResourceKeys.TabbedMdiContainerTabItemBackgroundActiveSelectedBrushKey}">

<LinearGradientBrush.GradientStops>

<GradientStopCollection>

<GradientStop Color="#9D9DA1" Offset="0" />

<GradientStop Color="#D9D9E1" Offset="1" />

</GradientStopCollection>

</LinearGradientBrush.GradientStops>

</LinearGradientBrush>

</Window.Resources>

<docking:DockSite x:Name="dockSite" CanDocumentWindowsRaft="True" Padding="0,0,0,1" >

<docking:SplitContainer>

<docking:Workspace >

<docking:TabbedMdiHost IsImageOnTab="true">

<docking:TabbedMdiContainer x:Name="TabsGroup" TabOverflowBehavior="Menu" >

<docking:DocumentWindow Title="WelcomeDocument.rtf" Description="Rich-text document"

FileName="C:\Users\Actipro\My Documents\WelcomeDocument.rtf">

</docking:DocumentWindow>

<docking:DocumentWindow Title="AnotherDoc" Description="Rich-text document"

FileName="C:\Users\Actipro\My Documents\WelcomeDocument.rtf" />

</docking:TabbedMdiContainer>

</docking:TabbedMdiHost>

</docking:Workspace>

</docking:SplitContainer>

</docking:DockSite>