As you can see the screencut above, you cannot know which pane is currently selected now,
So please apply a deep enough background color (ex. black or #1f1f1f) for current selected pane.
The key point is “User cannot know which pane is currently working now”.
When you see the figure above, you might guess “where am I?”,
because you don’t know which pane is selected by you. but if it change to the figure below, you will know “I am on 2nd one” right now:
That is what I mean about “deep background color for selected pane”.