MediterraneanNight is a good-looking pack of GTK 3 themes, it contains 12 themes for Gnome 3.6.x and Gnome 3.8.x. They are Mediterraneandark, Mediterraneandarkest, Mediterraneangraydark, Mediterraneanlight, Mediterraneanlightdarkest, Mediterraneannight, Mediterraneannightdarkest, Mediterraneantribute, Mediterraneantributeblue, Mediterraneantributedark, Mediterraneanwhite, Mediterraneanwhitenight.
Here are the screenshots:
To install the theme, first download theme pack at gnome-look.org:
Unpack the downloaded package and put all extracted under /home/USER/.themes folder (create .themes if not exist).
Then you can choose them using Gnome Tweak Tool, Unity Tweak Tool (available in Software Center).