Any idea where I can find the css page this is based off of?
What do you mean "based off of"?
That CSS page provides custom styling for links and avatars of the wiki's staff. It is imported into the wiki's MediaWiki:Common.css.
Is there a Fandom Developers or Community Central page that I can get this from? Or?
It's a custom CSS page, not a preset. The page you linked is the original.
You can insert similar CSS into your wiki's MediaWiki:Wikia.css page that you want specific to your wiki.
What do you think?