I wrote this userscript last year while working on a project.
// ==UserScript==
// @name Wanikani Hiragino Font
// @namespace rfindley
// @version 1.0
// @description Display Hiragino font on Wanikani
// @author Robin Findley
// @match https://www.wanikani.com/*
// @grant none
// ==/UserScript==
(function() {
var css = '[lang="ja"] {font-family: ".Hiragino Kaku Gothic Interface W5 Medium"; font-weight:100;}';
$('head').append('<style type="text/css">'+css+'</style>');
})();
Just change it to the font you want to use.
(Even though this actually uses Hiragino, you may need to adjust the font name slightly, because I installed Hiragino in an unusual way on my system)