I use a simple bookmarklet in such situations:

    javascript:(function(){w=window;d=w.document;b=d.querySelector('body');s=d.createElement('style');s.innerHTML='body{margin:0 auto!important;width:100%;max-width:120ch;font:normal 18px/1.5 Helvetica,Calibri,Arial,sans-serif;color:#333;background:#f9f9f9;}main,#main{margin:0 auto!important;}';b.setAttribute('style','');b.append(s);})();

It's pretty basic - changes body width to 120ch, centers body and main, and updates body font. Despite some issues, works great for a one-click fix.