blue GTK+ theme from the Shimmer Project

 Bluebird was the default theme for Xubuntu 10.10 but since 2011, Greybird
 has been Xubuntu's default theme. As its name indicates, Bluebird is a
 blue theme. The Shimmer Project are the designers and developers of both
 Bluebird and Greybird.
 .
 Although originally created for Xfce, Bluebird also supports other
 GTK+-based environments including Cinnamon, GNOME Shell, MATE, and Unity.