This css colorizes some file types' icons, so different files can be distinguished more easily. It also changes some icons to make distinguishing them even more easy.
You can also set this on the commandline.
Since no one else showed exactly how to do this, I will be kind and explain it for you. Since I am using a docker image of Filebrowser, it uses the 'srv' directory as the volume mount for the file directory.
I created a new folder in my root Filebrowser called custom then placed the custom.ss file there.
It's important to leave the trailing slash so the path can call and append the custom.css file.
Once you save this path, you might need to clear your browser cache to see the changes.