How to reproduce. 1. Install yakuake 2. set scrollbar to "hide" 3. set "color scheme & background" to "Breeze" or "Dark Pastels" and split the terminal horizontally or veritically. The separator line is not visible. If you use a black background, you can see the separator line. There should be a option to change the color and/or the width of this line.
I would also very much like to be able to configure the color of the divider. For me it's a dark grey and quite difficult to see between black terminals. I tried changing themes and restarting but that didn't seem to change it.
+1! I find the line very difficult to see especially once the screens are filled with text. Having the line be configurable (size/colour) would really help the legibility.