wget http://download.opensuse.org/repositories/home:Horst3180/xUbuntu_15.10/Release.key
sudo apt-key add - < Release.key

sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_15.10/ /' >> /etc/apt/sources.list.d/arc-theme.list"
sudo apt-get update
sudo apt-get install arc-theme

wget http://download.opensuse.org/repositories/home:Horst3180/xUbuntu_15.04/Release.key
sudo apt-key add - < Release.key

sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_15.04/ /' >> /etc/apt/sources.list.d/arc-theme.list"
sudo apt-get update
sudo apt-get install arc-theme