classic-theme-restorer software package provides virtual package provided by xul-ext-classic-theme-restorer, 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 classic-theme-restorer
classic-theme-restorer is installed in your system.
Make ensure the classic-theme-restorer package were installed using the commands given below,
$ sudo dpkg-query -l | grep classic-theme-restorer*
You will get with classic-theme-restorer package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | classic-theme-restorer |
| Version | |
| Description | virtual package provided by xul-ext-classic-theme-restorer |
| Sub Section | Virtual |