Install gtk3-engines-unico by entering the following commands in the terminal:
sudo apt update sudo apt install gtk3-engines-unico
Description:
Unico Gtk+ 3 theme engine
Unico is a Gtk+ engine that aims to be the more complete yet powerful theming engine for Gtk+ 3.0 and newer. It’s the first Gtk+ engine written with Gtk+ style context APIs in mind, using CSS as first class citizen.
Homepage: https://launchpad.net/unico
Version: 1.0.3+14.04.20140109-0ubuntu1
Section: universe/x11