Change font-weight for all attribute lang-ja in the document. Use with a script injector extension, such as Tampermonkey for Chrome or GreaseMonkey for Firefox.

KanaEagleVision

// ==UserScript==
// @name       KanaEagleVision
// @namespace  http://www.wanikani.com
// @version    0.1
// @description  Changes font-weight to  normal for anything attributed with lang=ja. jQuery inject script from https://gist.github.com/eristoddle/4440713
// ==/UserScript==    

function addJQuery(callback) {
    var script = document.createElement("script");
    script.setAttribute("src", "//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js");
    script.addEventListener('load', function() {
        var script = document.createElement("script");
        script.textContent = "window.jQ=jQuery.noConflict(true);(" + callback.toString() + ")();";
        document.body.appendChild(script);
    }, false);
    document.body.appendChild(script);
}
function fontWeightChange(){
    $(document).ready(function() {
      $('[lang=ja]').css('font-weight', 'normal');
    });
}
// load jQuery and execute the main function
addJQuery(fontWeightChange);