Clearlooks is a theme engine for GTK+, the main widget toolkit used by the GNOME desktop environment. It is based on Red Hat's Bluecurve theme. The theme engine draws the actual widgets, and can be configured with "themes" that invoke the theme engine with different configurations. It was the default theme for GNOME since version 2.12 until GNOME 3 when it was replaced by Adwaita. Many users have contributed themes that have changed the colors and some visual effects, leading to many derivative themes.
The creators of the Clearlooks GTK+ engine were Richard Stellingwerff and Daniel Borgmann; however, since 2005 the engine has been developed by GNOME, and current developers are Andrea Cimitan and Benjamin Berg.
Qt, versions 4.2 to 4.4, use a port of Clearlooks called Cleanlooks to better integrate with GTK applications.