I ran into the same problem, I want to style the buttons (background, icon etc.). I didn't find a way to do so.

But: with this rule I could disable the scroll buttons which in turn (re)enabled the traditional scrollbars which can be styled rather easily

QComboBox {
  combobox-popup: 0;
}

Found here https://forum.qt.io/topic/152637/qcombobox-unkown-top-and-botton-scrollers

FWIW apparently this depends on the style/theme of the operating system, and also on the window manager on Linux, it seems.