Numix is a modern flat theme with a combination of light and dark elements. It supports Gnome, Unity, XFCE and Openbox.
Numix GTK theme is a part of the Numix Project http://numixproject.org
If you have few bucks to spare, please consider purchasing the theme to support the project and show that you appreciate the work and time spent into making the theme.
As a bonus, you'll also get an additional Numix Dark theme with the purchase.
Ubuntu users can install Numix from our PPA using the following commands,
sudo add-apt-repository ppa:numix/ppa
sudo apt-get update && sudo apt-get install numix-gtk-theme
Arch Linux users can install Numix from the community repo
pacman -S numix-themes
Fedora and OpenSUSE users can install Numix from our build service repo - https://build.opensuse.org/project/show/home:paolorotolo:numix
Extract the zip file to the themes directory i.e. /usr/share/themes/
To set the theme in Gnome, run the following commands in Terminal,
gsettings set org.gnome.desktop.interface gtk-theme "Numix"
gsettings set org.gnome.desktop.wm.preferences theme "Numix"
To set the theme in Xfce, run the following commands in Terminal,
xfconf-query -c xsettings -p /Net/ThemeName -s "Numix"
xfconf-query -c xfwm4 -p /general/theme -s "Numix"
GTK+ 3.6 or above
Murrine theme engine