I see, sorry I didn’t read the entire thing but rather scanned it.
Looks like this author is using both TamperMonkey scripts AND custom CSS, which is, of course, possible, as having custom JavaScript is very powerful. Someone even made an entire enhancement to Dynalist using JavaScript.
JavaScript is not as straightforward as CSS though, so you’d need to do some tinkering on your end. Unfortunately, unlike some other use cases, changing font on an item level isn’t something people have done and shared on the forum before.