Black bg chat text for baby

tumbblkbg.js

// ==UserScript==
// @name         Black bg chat text for baby
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Black bg chat text
// @author       Gravin
// @match        https://www.tumblr.com/*
// @icon         https://www.google.com/s2/favicons?domain=tumblr.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    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('.CX_9D { background-color: #000 !important; color: white !important; }');
})();