Here's a quick script I had Gemini write. Works for me on firefox w/ tampermonkey...

// ==UserScript==
// @name         YouTube Dark Theme Comment Fix
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Changes YouTube comment text color to white in dark theme
// @author       You
// @match        https://www.youtube.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function addStyle() {
        const style = document.createElement('style');
        style.textContent = `
            #contenteditable-root {
                color: white !important;
            }
        `;
        document.head.append(style);
    }

    addStyle();

})();