Styling Floating windows in TabbedMdi

I can't figure out what style to use or how to change the background color of the floating windows when using DockSite with TabbedMdi

<Style TargetType="{x:Type docking:DockSite}">
    <Setter Property="Background" Value="#293955" />
</Style>

The DockSite style works for the main window, but not the floating window.

<Style TargetType="{x:Type docking:Workspace}">
   <Setter Property="Background" Value="Red" />
</Style>

Almost works, except for the approx 5-10px margin around the DockSite.