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<Grid>
        <Menu Height="23" HorizontalAlignment="Stretch" Name="menu1" VerticalAlignment="Top" >
            <Menu.Background>
                <ImageBrush ImageSource="/WpfApplication1;component/Images/Menubackground.png" />
            </Menu.Background>
            <MenuItem Header="Test 1" 
                      MouseEnter="MenuItem_MouseEnter"
                      MouseLeave="MenuItem_MouseLeave"
                      Click="MenuItem_Click" 
                      BorderThickness="0"
                      DataContext="{Binding}" 
                      Name="cmdMenuItem1" > 
                <MenuItem Header="Test 3" />
            </MenuItem>
        </Menu>
    </Grid>					

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 private void MenuItem_MouseEnter(object sender, MouseEventArgs e)
        {
            cmdMenuItem1.Background = Brushes.Yellow;
            cmdMenuItem1.BorderBrush = Brushes.Orange;       // <-- Keine veränderung
            cmdMenuItem1.BorderThickness = new Thickness(3); // <-- Ebenfalls keine Veränderung
        }