Это хорошо сработало для меня.
<Style x:Key="TransparentStyle" TargetType="{x:Type Button}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Border>
<Border.Style>
<Style TargetType="{x:Type Border}">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="DarkGoldenrod"/>
</Trigger>
</Style.Triggers>
</Style>
</Border.Style>
<Grid Background="Transparent">
<ContentPresenter></ContentPresenter>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Button Style="{StaticResource TransparentStyle}" VerticalAlignment="Top" HorizontalAlignment="Right" Width="25" Height="25"
Command="{Binding CloseWindow}">
<Button.Content >
<Grid Margin="0 0 0 0">
<Path Data="M0,7 L10,17 M0,17 L10,7" Stroke="Blue" StrokeThickness="2" HorizontalAlignment="Center" Stretch="None" />
</Grid>
</Button.Content>
</Button>