@avrratings A bit late so no idea if you still are looking for a fix but I'm gonna reply anyway in case someone in the future is looking for it. Its not working because the scripts is running before those elements have been initialized. An easy fix is not not remove the elements but just add a css style to make the white glow not be displayed. This should work :

// ==UserScript==
// @name Remove white hue from dark mode player
// @namespace http://tampermonkey.net/
// @version 0.1
// @description try to take over the world!
// @author You
// @match https://www.youtube.com/*
// @icon https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant none
// @require https://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js
// ==/UserScript==

(function () {
'use strict';
var sheet = document.createElement('style');
var sheet2 = document.createElement('style');

sheet.innerHTML = ".ytp-gradient-top {display: none !important};"; sheet2.innerHTML = ".ytp-gradient-bottom {display: none !important};"; document.body.appendChild(sheet); document.body.appendChild(sheet2);