@wrekler you have to set the margin and spacing of the QLayout item to 0 as well, not only in the StyleSheet. I believe those are 2 different pairs of shoes.
To clarify what @J-Hilk is saying (which I believe to be correct): a QLayout is not a QWidget and so cannot have its attributes affected via a stylesheet rule (sadly). That has to be done via explicit, back-end code.
Here we go! That was the problem! Layout cannot be affected by a stylesheet file! Now I know how styling layout works! Thank!