when the display area in the pane is larger than the displayed columns the area is displayed in a different color. Note in the image there are two columns displayed, Name and Modified, and the area to the left is empty and themed in gray. Seems this should be the same background color as the rest of the pane.
It affects the inactive pane only. It is part of the Colors and Fonts > File Display Background > Destination background. Seems this setting should affect the entire pane not just the empty column.