Install albatross-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install albatross-gtk-theme
dark and light GTK+ theme from the Shimmer Project
Albatross was the default theme for Xubuntu 9.10 and 10.04 LTS. Albatross uses dark menus but light-colored content areas. The Shimmer Project are the designers and developers of this theme. . Albatross works best in Xfce since it currently has incomplete support for newer versions of GTK+ 3.