gtk-theme-switch software package provides GTK+ theme switching utility, 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 gtk-theme-switch
gtk-theme-switch is installed in your system.
Make ensure the gtk-theme-switch package were installed using the commands given below,
$ sudo dpkg-query -l | grep gtk-theme-switch *
You will get with gtk-theme-switch package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | X Window System software |
| Package | gtk-theme-switch |
| Version | 2.1.0-5 |
| Description | GTK+ theme switching utility |
| Sub Section | X272 |