In the Default Shutdown.xml you can edit the wording in following section. I am not skilled in code and I was able to change the text, color, size, and add an italic easily.
<Grid Grid.Row="5" Grid.Column="0" Grid.ColumnSpan="5">
<TextBlock Text="Thank You For Playing..." Foreground="Aqua" HorizontalAlignment="Center" VerticalAlignment="Center"
FontSize="80" FontStyle="Italic" Typography.Capitals="AllSmallCaps" />
For my son he wanted a standard exit image of Sub-Zero so I was even able to edit the theme to use a default image only. This is for his minicade in his room.