Set splitter background and size

SplitterBackground is the property used to apply background to splitter between docked Children.

XAML

<syncfusion:DockingManager Name="DockingManager" SplitterBackground="Black">    

	<Grid Name="grid1" syncfusion:DockingManager.Header="Window1"/>    

	<Grid Name="grid2" syncfusion:DockingManager.Header="Window2"/>

</syncfusion:DockingManager>

C#

DockingManager.SplitterBackground = Brushes.Black;