Ok, let's do a checklist to make it work:
1) You probably downloaded the .zip from github, right?
2) Uncompress the master.zip, you will get a folder named "iris-light-master"
3) Copy that folder in /usr/share/themes/
3a) But be careful! you will need root access to do that: run (pressing alt+f2) gksu nautilus
(enter your password) and finally browse to /usr/share/themes/ and paste the folder.
Or you can use an script that I made:
In one move, you should be able to download, install and apply the theme by pasting this code in your terminal:
Iris Light theme:
mkdir -p ~/.iris-install && cd ~/.iris-install && wget https://github.com/xyl0n/iris-light/archive/master.zip && unzip master.zip && sudo cp -a iris-light-master /usr/share/themes && gsettings set org.gnome.desktop.interface gtk-theme 'iris-light-master' && cd ~ && rm -R ~/.iris-install
Iris Dark theme:
mkdir -p ~/.iris-install && cd ~/.iris-install && wget https://github.com/xyl0n/iris/archive/master.zip && unzip master.zip && sudo cp -a iris-master /usr/share/themes && gsettings set org.gnome.desktop.interface gtk-theme 'iris-master' && cd ~ && rm -R ~/.iris-install
Yes yes, just copy and paste in your terminal. It will be automatically applied. :)