I don’t think I ever posted this here. As I am bad at keeping track of it myself, adding this here as my backup solution.
The following tampermonkey script will give the site a wonderful font for reading. You could change it to another font too, I suppose, but I mean… why?
// ==UserScript==
// @name QT3 Fonts
// @namespace http://tampermonkey.net/
// @version 0.1
// @description solid font for old man eyes
// @author daagar
// @match https://forum.quartertothree.com/*
// @grant GM_addStyle
// ==/UserScript==
(function() {
'use strict';
var link = document.createElement('link');
link.rel = 'stylesheet';
// See https://fonts.google.com for all the fonts you can put here!
link.href = 'fonts.googleapis.com/css2?family=Roboto:[email protected];500&display=swap'
document.getElementsByTagName("head")[0].appendChild(link);
GM_addStyle('html { FONT-FAMILY: Roboto; !important;font-weight: 500 !important;.list-controls .category-dropdown-menu a.badge-category,.list-controls .category-dropdown-menu .dropdown-header { font-family: Roboto !important; } .badge-wrapper { font-family: Roboto !important;} aside.onebox .onebox-body h3, aside.onebox .onebox-body h4, .category-list tbody .category h3, .names span.first { font-weight: normal !important;} }');
})();
EDIT: Actual working version (or at least what I consider working) thanks to @fox.ferro CSS skillz.