gray-theme software package provides Gray GTK theme, 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 gray-theme
gray-theme is installed in your system.
Make ensure the gray-theme package were installed using the commands given below,
$ sudo dpkg-query -l | grep gray-theme *
You will get with gray-theme package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | X Window System software |
| Package | gray-theme |
| Version | 0.1-0ubuntu2 |
| Description | Gray GTK theme |
| Sub Section | X259 |