Install gtk2-engines-blueheart by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-blueheart
Description:
Blueheart GTK+ 2.x Theme
BlueHeart is a port to GTK2 of the original famous BlueHeart theme by Claudiu Christian Fofiu. Permission granted.
Version: 0.20070123-0ubuntu2
Section: universe/misc