4
Install gnome-tweak-tool
sudo apt-get install gnome-tweak-tool
Download the theme from Gnome-art(make sure that it is gtk3 theme)
Extract the theme and copy it to the .themes(.icons for icon themes) folder in your home folder (It is hidden by default press ctrl+h to see it
open advanced settings from unity dash
Move to 'Theme' option on the left sidebar. You can now change the Window theme,icon theme and gtk theme from there by changing the options in the dropdown list