In the meantime, following muru's suggestion I created (with a bit of help from jokerdino♦ and Seth♦) a userscript which underlines all the links on askubuntu.com:

<!-- language: js -->

    // ==UserScript==
    // @name         New Userscript
    // @namespace    http://tampermonkey.net/
    // @version      0.1
    // @description  try to take over the world!
    // @author       You
    // @match        http://askubuntu.com/*
    // @match        http://meta.askubuntu.com/*
    // @grant        none
    // ==/UserScript==
    /* jshint -W097 */
    'use strict';
    
    $('a').css({"text-decoration": "underline"});

The result is pretty good.

To install it simply install Greasemonkey (Firefox / Firefox derivatives) or Tampermonkey (Chrome / Chromium), add the script and enable it.

Known bugs:

 - It doesn't work for content loaded after the page was loaded the first time (new active questions, edits etc.)