breeze-icon-theme software package provides Default Plasma icon 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 breeze-icon-theme
breeze-icon-theme is installed in your system.
Make ensure the breeze-icon-theme package were installed using the commands given below,
$ sudo dpkg-query -l | grep breeze-icon-theme *
You will get with breeze-icon-theme package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | KDE |
| Package | breeze-icon-theme |
| Version | 4:5.18.0-0ubuntu1 |
| Description | Default Plasma icon theme. |
| Sub Section | KDE |