You can call
_presenter.SetBorderAndTitleBar(false, false);
You can change the transparency with SetLayeredWindowAttributes
You can change the background with a Grid, a Border and Border.Background
For example, I have set a Gradient as background in the XAML :
<Border.Background>
<LinearGradientBrush StartPoint='0,0' EndPoint='1,1'>
<GradientStop Color='Yellow' Offset='0.0' />
<GradientStop Color='Red' Offset='0.25' />
<GradientStop Color='Blue' Offset='0.75' />
<GradientStop Color='LimeGreen' Offset='1.0' />
</LinearGradientBrush>
</Border.Background>
I get :