Customizing Mahapps.MetroWindow Close Button

Here is an inherited custom style for a close button with mouse over / pressed effect:

<Style x:Key="MetroWindowCloseButtonStyle"
       TargetType="{x:Type Button}"
       BasedOn="{StaticResource MetroWindowButtonStyle}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Button}">
                <Grid x:Name="grid"
                      Background="{TemplateBinding Background}">
                    <ContentPresenter x:Name="contentPresenter"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      Margin="{TemplateBinding Padding}"
                                      VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                      SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                      RecognizesAccessKey="True"
                                      Opacity="0.75" />
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsMouseOver"
                             Value="True">
                        <Setter TargetName="contentPresenter"
                                Property="Opacity"
                                Value="1" />
                        <Setter TargetName="grid"
                                Property="Background"
                                Value="#E04343" />
                    </Trigger>
                    <Trigger Property="IsMouseOver"
                             Value="False">
                        <Setter TargetName="contentPresenter"
                                Property="Opacity"
                                Value=".5" />
                    </Trigger>
                    <Trigger Property="IsPressed"
                             Value="True">
                        <Setter TargetName="grid"
                                Property="Background"
                                Value="#993D3D" />
                    </Trigger>
                    <Trigger Property="IsEnabled"
                             Value="false">
                        <Setter Property="Foreground"
                                Value="#ADADAD" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

The original (obsolete) style can be found here on GitHub

Hope that helps.