One of the main reasons I subscribed to Squarespace was so I could host my portfolio quickly without having to write in a bunch of code, so unfortunately I'm not 100% sure I understand what you mean by "Using Custom CSS, wrap your elements inside the collection ID for each page that you want to change." I downloaded that browser extension to be able to see the part of the code I need to edit, but it doesn't seem to work.
When I go through Home >Design >Custom CSS I entered the code as such:
#collection-617848c233fecf2977e89692 {
h1,h2,h3 {
font-family: 'Bebas Neue', cursive;
}
}
which for some reason inexplicably ended up looking like Comic Sans despite Bebas Neue being included in the default library..