@Valer I think we need to understand what this plugin is doing.
CSS isn’t something we’ve allowed to be injectable though it could be.
You can still use your own CSS but that’s more at the FOG Configuration -> FOG Setting -> FOG_THEME, but you would need to put it on your server in a location you type the path too here.
https://docs.fogproject.org/en/latest/development/plugin-development
This is a good toolkit for understanding how to build your own plugin.