How to stop NSOutlineView transparent background

I have NSOutlineview top and bottom - this is a custom view in one window. I want to change the color of this custom view to have the same color with NSOutlineview.

However, I cannot do this as the background color of the NSOutlineView appears to be gray, but it is also transparent, makes its color different if there is another application window below it.

How do I make NSOutlineview and my custom view the same color?