Obsolete - Messenger.com FINALLY implented a dark mode! Dark mode for messenger.com, the code is already there as you can see. Facebook just haven't gotten around to implementing it in the UI yet..

NativeDarkMessenger.user.js

// ==UserScript==
// @name         Native Dark Messenger
// @namespace    https://github.com/Damperen
// @version      1.9.5
// @description  Almost Native dark mode for Messenger
// @author       https://github.com/Damperen
// @match        https://www.messenger.com/*
// @grant        none
// @run-at       document-idle
// @icon         https://i.imgur.com/VNn3FEX.png
// ==/UserScript==

(async function() {
    'use strict';

    var mutationObserver = new MutationObserver(function(mutations) {
        mutations.forEach(function(mutation) {
            mutation.addedNodes.forEach(function(node) {
                if(node.nodeType === 1){
                    setDarkModeMultiple(document.querySelectorAll(".__fb-light-mode"));
                    document.querySelectorAll(".qbubdy2e.pmk7jnqg>svg").forEach(function(svg){
                        svg.style.fill = "#242526"
                    });
                }
            });
        });
    });

    mutationObserver.observe(document.documentElement, {
        childList: true,
        subtree: true,
    });

    function setDarkModeMultiple(obj){
        if(!document.documentElement.className.contains('__fb-dark-mode')) {
            document.documentElement.className = '_9dls __fb-dark-mode';
            const sheet = new CSSStyleSheet();
            sheet.replaceSync('.a8c37x1j, path{fill: #fff} .__fb-dark-mode{--header-height: 0px} :root{color-scheme: dark} #facebook{ overflow: hidden!important} div, span>a{ color:white!important;} ');
            document.adoptedStyleSheets = [sheet];
        }

        obj.forEach(function(item) {
            item.className = item.className.replace('light','dark');
        });
    }
})();

Tampermonkey 4.11.6120 (5 November 2020), Mozilla Firefox 86.0 (64-bit) - the script is not offered as available for installation.

WELL! I finally figured out how to do this.. Sigh.. I only needed to change the script name from NativeDarkMessenger.js to NativeDarkMessenger.user.js.
I learned something new! Thank you.

Removed the base64 encoded messenger.com favicon image, it was screwing with the search on github, the gist could not be found at all in the search results.

Possibly fixed the error mentioned in the previous comment.

Now the script installation interface appears, thanks for the fix!
Tampermonkey 4.12.6130, Microsoft Windows 10.0.19042.844

Possibly fixed the error mentioned in the previous comment.

Now the script installation interface appears, thanks for the fix!
Tampermonkey 4.12.6130, Microsoft Windows 10.0.19042.844

You're very welcome, thank you for notifying me! :)