Bumping this. There is no "themed icons" toggle in the Wallpaper and style settings menu.
Stock android seems to have support for disabling it (but not sure really), could we have this in GOS too?
edit: so there IS such an option. Just need to swipe right in that menu, and then there's an Icons -> Themed icons toggle. And it apparently does work as expected: the apps' original/native colored icons are displayed instead of the ugly monochrome themed ones. Of course it may work differently in other launchers, it's up to them.