1) Where to put your CSSURL copied
Folder-level global CSSURL copied
Place a global.css in a content folder when the folder needs to style the whole page shell.
Vyasa links that file as a normal stylesheet for every page in that folder subtree. That means it can safely define rules for:
html, body
#page-container, #site-navbar, #posts-sidebar, #toc-sidebar
@font-face
@keyframes
:root custom properties
This is the correct tool for book themes, app-like section chrome, or folder-specific fonts.