I assume you’ve downloaded theme from gnome-look or other site. If not, you’ve to download new theme from gnome-look site.
To install a new icon theme,download the Icon theme tarball. Enter the following command in terminal.
cp tarball.tar.gz /home/username/.icons
cd /home/username/.icons
tar xfz tarball.tar.gz
Goto Computer > Desktop Preferences > Themes to change to the new Icon theme.
If you do not have a .icons directory then you’ve to create one. Use the command shown below.
mkdir -p /home/username/.icons
