Libra is one of the best theme derived from flattastic theme suite. This theme is light with combination of blue color and it has menu colored buttons (minimize, maximize, close). It features a modern and clean look, with fully integrated and tested support for many of the popular desktops Including: Unity, Gnome Shell, Cinnamon, Mate, Gnome Classic & Fallback. It has been designed to be compatible with most GTK2/3 desktops out of the box. Rave-X-Colors and Ursa icons used in following screenshots. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes.
Update: New dark version released for 15.04 Vivid
To install Libra Gtk theme in Ubuntu 15.04/14.10/14.04/Linux Mint 17.1/17 open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal: