Missing Twitter's dark navigation bar? Same here. Fixed.

twitter-dark-navbar.user.js

// ==UserScript==
// @name        Twitter Dark Navbar
// @namespace   http://example.com
// @description hacked with love by @akuhn
// @include     https://twitter.com/*
// @version     1
// @grant       none
// ==/UserScript==

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

addGlobalStyle(".global-nav-inner { border-color: #505050; " +
    "background-image: linear-gradient(to bottom, #393939, #282828); " +
    "box-shadow: 0 1px 10px rgba(0, 0, 0, 0.1); }");