10C_LightDark_Toggle.user.js

// ==UserScript==
// @name        10C Light/Dark Toggle
// @namespace   http://mikedeep.com
// @description Adds back the light/dark toggle switch on the Pearl Jam Forum.
// @include     http://community.pearljam.com/*
// @version     1
// @grant       none
// ==/UserScript==

var brightness = readCookie('dark');
if(brightness > 0) { toggleCSS(); }

function addGlobalStyle(css) {
    var head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    var style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
}

var brightnessToggle = document.createElement('span');
brightnessToggle.className = "ToggleFlyout";
document.getElementsByClassName('MeMenu')[0].appendChild(brightnessToggle);

var brightnessAnchor = document.createElement('a');
brightnessAnchor.className = "MeButton FlyoutButton";
brightnessToggle.appendChild(brightnessAnchor);
brightnessAnchor.href = "#";
brightnessAnchor.title = "Switch to Dark Theme";

var brightnessSpan = document.createElement('span');
brightnessSpan.className = "Sprite Sprite16 SpOptions userScriptToggle";
brightnessAnchor.appendChild(brightnessSpan);

brightnessAnchor.addEventListener('click', toggleCSS);

function toggleCSS() {
	var darkClass = "Theme-Dark";
	var checkClass = document.body.classList.contains(darkClass);
	if(checkClass > 0) {
		document.body.classList.remove(darkClass);
		document.cookie = "dark = 0; max-age = 31536e3; path=/";
	} else {
		document.body.classList.add(darkClass);
		document.cookie = "dark = 1; max-age = 31536e3; path=/";
	}
}

addGlobalStyle (
"body.Theme-Dark {" +
"   background-color: #000;" +
"}" +
".Theme-Dark .Navbar .Menu .Brand {" +
"   -webkit-filter: invert(100%);" +
"   -moz-filter: invert(100%);" +
"   filter: invert(100%);" +
"}" +
".Theme-Dark .Navbar .Menu a," +
".Theme-Dark .Toolbar .Menu li a {" +
"   color: #fff;" +
"}" +
".Theme-Dark .Navbar .Menu a:hover {" +
"   color: #a88231;" +
"}" +
".userScriptToggle {" +
"   background-position: -320px -100px;" +
"}" +
".Theme-Dark .userScriptToggle {" +
"   background-position: -320px -120px;" +
"}" +
".Theme-Dark .TagCloud li a {" +
"   color: #fff;" +
"}"
);

// quirksmode.org/js/cookies.html
function readCookie(name) {
	var nameEQ = name + "=";
	var ca = document.cookie.split(';');
	for(var i=0;i < ca.length;i++) {
		var c = ca[i];
		while (c.charAt(0)==' ') c = c.substring(1,c.length);
		if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
	}
	return null;
}