Install darkcold-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install darkcold-gtk-theme
Description:
dark GTK2/GTK3/Metacity theme
A night-style window theme by OriginalSeed. It is mostly black, with dark blue elements.
Homepage: https://github.com/kilobyte/darkcold
Version: 1.0-2
Section: universe/misc