QTab with QScrollArea in CSS, strange behavior

[Papercut]

Hi All,
I've a weird behavior on my simple qt app.
I'm trying to set color of all widget in the GUI.
Unfortunately i'm not able to set the color of the background of the two rect (one on the left side of the first tab, and the other one on the right side of the last tab).
This is the screenshot of the problem screenshot.
Thanks for any suggestion.