Adwaita is the default gtk theme for the GNOME 3 desktop. This Atom UI theme is a derivative of the atom-light-ui theme that adds integration with the Adwaita theme. Currently it changes the tab style, the scrollbars and the tree view.
Many current (as of 8 July 2014) distributions use older versions of the GNOME 3 desktop, which used a different, trapezoidal tab shape. I chose to use the newer rectangular tab shape that comes with new versions of GNOME, such as the version currently available in the official Arch Linux repositories, 3.12.