Install gtk2-engines-xfce by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-xfce
Description:
GTK+-2.0 theme engine for Xfce
This package contains a theme engine allowing users to apply various GTK2.0 themes to Xfce. It also contains some ready themes, but users are of course free to use other themes or design their own.
Homepage: http://www.xfce.org/
Version: 3.2.0-3
Section: universe/x11