As far as I know, Fibery doesn’t support custom templates (based on an html/css format). This has been on my wish list for a while now and hoping one day it would be possible (see PDF export of documents).
I can imagine that the Fibery team would want to have a no-code template system, but having an html based system (a la pdfmonkey.io) would be sufficient for most fibery power users who otherwise have to go through the trouble of creating automations to generate pdfs externally.
In any cased, I’ve provided an example of how I use pdfmonkey in that thread in case it is helpful.