greybird-gtk-theme software package provides Greybird GTK2 and GTK3 themes, you can install in your Ubuntu 16.04 LTS (Xenial Xerus) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install greybird-gtk-theme
greybird-gtk-theme is installed in your system.
Make ensure the greybird-gtk-theme package were installed using the commands given below,
$ sudo dpkg-query -l | grep greybird-gtk-theme *
You will get with greybird-gtk-theme package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | X Window System software |
| Package | greybird-gtk-theme |
| Version | 2.1.2-0ubuntu1 |
| Description | Greybird GTK2 and GTK3 themes |
| Sub Section | X261 |