If you want to modify the CSS and Javascript of VIMP, you need to concatenate those CSS and JS files afterwards.
For concatenation, nodejs and npm must be installed on your server.
For Debian 10 Buster it should be as simple as:
sudo apt-get install nodejs npm grunt npm i node-sass
For other distributions, please refer to https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions-enterprise-linux-fedora-and-snap-packages.
Read more about how to concatenate CSS and JS files in this article.
