It does not appear that @bridouz needs to inject custom CSS, but rather include a relevant script on select pages in order to embed a HTML-system for commenting.
That is the simplest way to go, as you do presumably not need or want the script to be loaded on all pages. Adding it to item.html.twig, for example, loads it only on pages using this template. If loaded generally to assets, it will be pipelined to all pages.
You could load it via assets and specify template-restrictions, but that is just a more difficult way to go leading to the same result.
They should be in the theme folder, as the root folder is fairly much a reserved folder for the system. Clearing cache could delete the script if located in root (if memory serves me right).