Gtk applications, like Midori, use gtk stock icons from selected theme to show actions WITHIN headerbars.
You can make a variant of the Numix theme to solve your issue.
First of all, create a new folder from terminal:
mkdir -p ~/.icons/mynumix
Copy the content of Numix theme icon folder to new one:
sudo cp -a /usr/share/icons/Numix/. ~/.icons/mynumix
Copy the gtk stock icons to "mynumix" folder:
sudo cp -a /usr/share/icons/elementary/actions/24/* ~/.icons/mynumix/24/actions/
Create icon cache for the new theme:
sudo gtk-update-icon-cache ~/.icons/mynumix
And finally, change to new theme:
gsettings set org.gnome.desktop.interface icon-theme 'mynumix'
To restore to default theme, just execute from terminal:
gsettings set org.gnome.desktop.interface icon-theme 'elementary'
You can combine sizes and themes changing the words '24' and 'elementary' by your favorite ones when you copy the stock icons.