Arc is a new GTK theme which looks amazing in my opinion. Many users were complaining about the complexity of installing the theme from source so the developer decided to add package repositories for Debian, Fedora, openSUSE and Ubuntu. The theme is also available from Arch Linux’s AUR.
Here you can find all package repositories: Arc Theme Repositories
To install Arc theme on Ubuntu / Linux Mint / elementary OS, run the following commands in Terminal:
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