A simple and hacky TamperMonkey script to convert Jira to dark mode. Sort of.

TMJiraDarkMode.js

// ==UserScript==
// @name         Jira Dark Mode
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Convert Jira to dark mode.
// @author       Peter Evans
// @match        https://*.atlassian.net/*
// @grant        none
// @require      https://cdn.walkme.com/player/resources/wmjQuery3315.js
// ==/UserScript==

function flipImgElements() {
    // Reset the smaller avatar images
    if (document.querySelector('.ghx-avatar-img')) {
        $('.ghx-avatar-img').each(function(i, el) {
            el.style.filter = 'invert(100%)';
        });
    }

    // Reset the bigger avatar and team images
    if (document.querySelector('span[role=img]')) {
        $('span[role=img]').each(function(i, el) {
            el.style.filter = 'invert(100%)';
        });
    }
}

function setBaseStyle() {
    // Invert the colour and set a dark background.
    let bodyEl = document.getElementsByTagName('body')[0];
    bodyEl.style.filter = 'invert(90%)';
    bodyEl.style['background-color'] = '#1f1f1f !important';
}


(function() {
    'use strict';

    setBaseStyle();

    // Avatar and team images need to be re-inverted once they load on the page.
    let observer = new MutationObserver(function(mutations) {
       flipImgElements();
    });

    observer.observe(document, {attributes: false, childList: true, characterData: false, subtree:true});

    console.log('JIRA Dark Mode enabled!');
})();