Numix is a modern flat theme with a combination of light and dark elements. It supports Gnome, Unity, and more.  

Screenshot :

Gnome 3.8 “Show Application” in numix theme with icons
Gnome 3.8 Browser , Music player and File Manager in numix theme with flat design
Numix theme in unity

Steps to install in Ubuntu 13.10:
**1. Open terminal                                                                                              

  1.  Add PPAsudo add-apt-repository ppa:numix/ppa
    sudo apt-get update 
    2.sudo apt-get install numix-gtk-theme numix-icon-theme numix-wallpaper-saucy