I use Ubuntu's toolbar theme (context) in my application which is black for Ambiance and grey for Radiance. So I have got two icon packs (a white and a black one), but how can I figure out which theme is used by the user? So I'll can write an if statement for it.
You can type in terminal:
gsettings get org.gnome.desktop.interface gtk-theme
Or in Python:
from gi.repository import Gio theme=Gio.Settings.new('org.gnome.desktop.interface').get_string('gtk-theme')
If you choose a stock icon, it will always use the icon of the theme which was chosen by the user.