Numix Circle theme is used as a fall-back theme for the Sardi icon collection. This means that if an icon is missing, Sardi will go look if an icon exists in Numix Circle. In order for this to work you should install Numix Circle.
Numix is a very professional looking theme and can be used on any distribution.
Various good links can be found on the net:
They have one thing in common.
They all use the ppa or personal package archives to download the icons. This has the advantage when the numix team updates their package that our linux mint updater will download the latest numix circle icons.
The structure is always similar but the name of the ppa is very personal
sudo add-apt-repository xxxxxx
Adding a place/repository to the download/update list
sudo apt-get update
Getting also the list of software, icons, themes, … of this new place/repository
sudo apt-get install xxxxxx
Installing the package you actually want.
In short: installing icons and theme
sudo add-apt-repository ppa:numix/ppa sudo apt-get update sudo apt-get install numix-icon-theme-circle numix-gtk-theme
You will see that numix-icon-theme will be installed as well. Numix-icon-theme-circle theme depends on the numix-icon-theme.
A tutorial has been made as part of the playlist “Linux Mint 18 Cinnamon a complete customisation with Sardi icons“.
Install Numix via scripts
You can use scripts if you want to.