Comment # 4 on bug 970653 from Wolfgang Bauer

Ok, those particular icons you mentioned are actually taken from the desktop
theme, not the icon theme. (although I'm not sure what you exactly mean with
"system tray", I'm assuming the icons displayed in the tray)

So change that.

But note that if the chosen desktop theme doesn't provide the corresponding
graphics, Plasma will fall back to the default theme which is breeze.