Вот немного доработанный мной вариант скрипта. Ссылки на каждом сайте будут подчеркиваться новым цветом!
// ==UserScript==
// @name Underline_links
// @namespace [url]http://tampermonkey.net/[/url]
// @version 0.1
// @description try to take over the world!
// @author You
// @include [url]https://*[/url]
// @include [url]http://*[/url]
// @grant none
// ==/UserScript==
(function() {
'use strict';
var i = Math.floor(Math.random()*8);
var elem = document.createElement('style');
switch (i) {
case 0:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: red !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 1:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: orangered !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 2:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: yellowgreen !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 3:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: green !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 4:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: teal !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 5:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: aqua !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 6:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: deepskyblue !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
break;
case 7:
elem.innerHTML ='a {line-height: 1 !important;display: inline-block !important;cursor: pointer;} a:after {display: block !important;position: relative !important;content: "" !important;height: 2px !important;width: 0% !important;background-color: fuchsia !important; transition: all .3s ease-in-out !important;left: 50% !important;} a:hover:after, a:focus:after {width: 100% !important;left:0% !important;} a:hover{text-decoration: none !important;}';
};
document.head.appendChild(elem);
})();
