hicolor-icon-theme software package provides default fallback theme for FreeDesktop.org icon themes, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install hicolor-icon-theme
hicolor-icon-theme is installed in your system.
Make ensure the hicolor-icon-theme package were installed using the commands given below,
$ sudo dpkg-query -l | grep hicolor-icon-theme *
You will get with hicolor-icon-theme package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Miscellaneous |
| Package | hicolor-icon-theme |
| Version | 0.15-1 |
| Description | default fallback theme for FreeDesktop.org icon themes |
| Sub Section | Miscellaneous |