ContextMenuのStyle変更

違うかもしれない

  <Style x:Key="{x:Type ContextMenu}" TargetType="{x:Type ContextMenu}">
   <Setter Property="Template">
    <Setter.Value>
     <ControlTemplate>
      <Border BorderBrush="Black" BorderThickness="3" CornerRadius="5">
       <StackPanel IsItemsHost="True" Background="Wheat"/>
      </Border>
     </ControlTemplate>
    </Setter.Value>
   </Setter>
  </Style>