QT_STYLE_OVERRIDE=gtk2 with Yaru doesn't obey custom gtk.css

[GeckoLinux]

Hi there, this report is with Yaru 22.10.3 on Debian 12. I set QT_STYLE_OVERRIDE=gtk2, and when used together with the Numix GTK theme it makes Qt5 apps obey the custom scrollbar width that I have set in ~/.config/gtk-3.0/gtk.css (I don't understand why it obeys that CSS when Qt is supposed to be imitating GTK2, not GTK3, but at any rate it works):

However, with Yaru it uses skinny scrollbars as defined in Yaru:

Is this by design, and is there any way I can override it? Here's my ~/.config/gtk-3.0/gtk.css :

scrollbar, scrollbar button, scrollbar slider {
	-GtkScrollbar-has-backward-stepper: true;
	-GtkScrollbar-has-secondary-backward-stepper: true;
	-GtkScrollbar-has-forward-stepper: true;
	min-width: 12px;
	min-height: 12px;
	border-radius: 0;
}