adwaita-icon-theme software package provides default icon theme of GNOME (small subset), 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 adwaita-icon-theme
adwaita-icon-theme is installed in your system.
Make ensure the adwaita-icon-theme package were installed using the commands given below,
$ sudo dpkg-query -l | grep adwaita-icon-theme *
You will get with adwaita-icon-theme package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | GNOME |
| Package | adwaita-icon-theme |
| Version | 3.18.0-2ubuntu3 |
| Description | default icon theme of GNOME (small subset) |
| Sub Section | GNOME |