Faenza was not updated for a long time, some icons are missing from the package. But don't worry, it is very easy to fix this issue.
You will have to edit faenza index.theme. You can find it in Faenza icon folder, e.g. /usr/share/icons/Faenza.
Open it as root
sudo gedit
/usr/share/icons/Faenza/index.theme
Locate 3th line from the top. It should look like that Inherits=gnome,hicolor
- Change it to
Inherits=ubuntu-mono-dark,gnome,hicolor
- Restart your theme with gnome-tweak-tools or just log out.
If ubuntu does not find suitable icon in faenza icon pack then it will fall back to default ubuntu icon.