Install gtk2-engines-cleanice by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-cleanice
Description:
CleanIce themes for GTK+ 2.x
This package contains a theme engine and 3 themes that allow you to configure the look of GTK+ 2.x-based programs. The programs need no modifications to use themes; the support is built in to GTK+ 2.x. . The Debian package provides these themes: CleanIce, CleanIce-Dark, CleanIce-Debian, CleanIce-Marble.
Homepage: http://sourceforge.net/projects/elysium-project
Version: 2.4.1-3
Section: universe/graphics