gtk2-engines binary package in Ubuntu Bionic amd64

 This package contains the "engines" that hide behind the themes for
 GTK+ and GNOME applications. They redefine the way GTK+ widgets are
 drawn. The package includes the following engines:
  * Clearlooks, the default GNOME theme, based on Bluecurve;
  * Crux, formerly known as the Eazel engine;
  * High contrast, which is used by some accessibility themes;
  * Industrial, the famous engine from Novell (formerly Ximian);
  * LighthouseBlue, another engine based on Bluecurve;
  * Metal, which gives a metallic look;
  * Mist, a flat and high performance engine;
  * Redmond95, which provides a look similar to that of Windows;
  * ThinIce.
 .
 A few sample themes using these engines are also included.

Source package