New Post: ChildWindow Inner Border

If your Window has the following properties set :
Background="Black"
BorderBrush="Black"
BorderThickness="0"
WindowBackground="Black"
WindowBorderBrush="Black"
WindowBorderThickness="0"
and you still see an inner Border, go in the "core/Themes/Aero2/WindowControl.xaml" (or "core/Themes/Generic/WindowControl.xaml") and change the "BorderThickness" in the following line from
<Border x:Name="ContentOuterBorder" Grid.Row="1" BorderBrush="Gray" BorderThickness="1">
from "1" to "0".