Install clearlooks-phenix-theme by entering the following commands in the terminal:
sudo apt update sudo apt install clearlooks-phenix-theme
GTK3 port of Clearlooks theme
Clearlooks-Phénix is a GTK3 theme which is a port of Clearlooks, the default theme for GNOME 2. Files for GTK3 is a Clearlooks theme ported to GTK3 CSS language. Files for GTK2 come from the unmodified Clearlooks theme.