You know you can fork Jitai from Github Gist, right?
Font styles would probably be mainly textbook style vs Mincho, but there are definitely others. I don’t know much about, “would font weight matter?”, though.
Some other fonts that might be considered, might include pixelated fonts; though, of course, it should be done in moderation.
Another thing that might matters, is - you should really wrap you UserScript code in IIFE, as not only that top-level code can be interfered, but also I don’t think anything other than var can be declared twice.