Sorry for being late on this. Submitted my dissertation and finally had the time to work on fixing the script. Many thanks to @rogerxiii who provided a quick fix in the meantime!
Jitai v3
I decided to completely rewrite the entire script.
Jitai is probably my most favorite script, so it’s sad to see that it is not anymore maintained by the original creator. Even before the recent WK update it wasn’t bug-free, and adding/removing fonts was ugly. You had to edit the script, but then you couldn’t receive updates anymore. So I wanted to work on something that makes it able to enable/disable fonts during reviews.
Changelog
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(New) Webfonts
Webfonts are now available, so you don’t have to install every font on your PC. If you already have the webfont on your PC, then it will prefer the installed font.
(The original script broke completely on Safari because of an update that prevents using local fonts. Likely other browsers will follow, see here why. Jitai v3 gets around this issue by introducing webfonts.) -
(New) Settings Panel
Fonts can now be enabled / disabled during review. It will only show you webfonts and local fonts available on your PC, so its appearance will slightly change based on what browser you use and what OS. @TreyE requested this.
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(New) Force a new random font with shortcut CTRL+J during review.
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Bugfix: Works now with DoubleCheckScript.
Previously there was a font bug when you retyped an answer, and then the hovering effect kept being inverted.
Important: If you do your reviews with Jitai v3 for the first time, please click on the
gear iconin the top left, thenSettings -> Jitaiand first enable the fonts you want to use!
Other notes
I tested it on latest Safari and Firefox. This is my first time writing a user script (even first time learning Javascript), so please be kind on me 
If there’s any bug, let me know o that we can work on a fix 
The main reason I worked on it was also to upload it on GitHub to make it Community-driven. That way I hope it is more likely to be updated, even if I finish WaniKani one day. That doesn’t mean I don’t intend to update it over time (still want to find a way to make ArmedBanana font work in Safari), but I probably won’t be able to work on everything immediately since I also need to focus on other work 
That said, you can already contribute on GitHub and add fonts you like (that’s after all the main thing what’s running Jitai!). I may write a proper guide on that sometime, but you are also free to make requests here.
~ ~ > Download here < ~ ~
~ ~ > Jitai on Github < ~ ~