power bi formatting group header in a matrix

[richdt]

I've been working on a matrix in power bi with group headers and sub group headers.
My issue is i cannot with the formatting panel in power bi draw a clear seperation line between my 1st group (Group kotoko1) and 2nd group (Group kotoko2), without drawing lines for all my columns!
So i tried to edit the background of each group with colors (see 2. Background color in image below)
All i want is to show visually a clear seperation between my 1st group (Group kotoko1) and 2nd group (Group kotoko2).
As per image below if i can do 1.Seperate line or 2. Background color on group, it will be great.

Thank all in advance for your help  

[lbendlin]

 please consider voting for the existing idea  at https://ideas.fabric.microsoft.com/?forum=2d80fd4a-16cb-4189-896b-e0dac5e08b41