Numix is popular theme created by satya, Numix Bluish is moded version of Numix GTK3 theme. It is kind of flat theme with the combination of dark and light elements, it supports Gtk 2/Gtk 3, Xfce, Unity, Openbox, and it also works well in Gnome Shell/Gnome Classic/Mate. Plastic-Colored icons used in screenshots. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes/icons.
To install Numix-Bluish theme in Ubuntu 14.04/13.10/Linux Mint 16 open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
sudo apt-get update
sudo apt-get install numix-bluish-theme