The library relies on these templates:
and their associated CSS stylesheet in /style.css.
There is also a Module:Number of works to display the (X works) mention in the cards.
Finally there is a template, Library_homepage, that contains the overarching CSS for the div containers that will contain the cards on the library homepage. That's all it does.
There is also some CSS in MediaWiki:Citizen.css specifically for the built-in dark mode.
It's otherwise pretty self-explanatory, especially when one looks at the user guide for the new library. Everything works with simple HTML and CSS.
In the library page, we have div containers for every heading and subheading. .library-container is used for the big cards and .library-container-mini is used for the mini cards.
The big cards are displayed in a flex style and the mini cards