Go to opera://flags/#enable-force-dark and put it on Default. Go to opera settings, at the bottom-ish, there's a setting called Force Dark Pages. Turn that off. Then go to youtube and turn on dark mode on youtube itself. Restart the browser and that fixed it for me

// ==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);

})();

$('.ytp-gradient-bottom').remove();
$('.ytp-gradient-top').remove();

Excuse my english I am italian

There are 2 things that cause this:

1. Forcing Opera's dark mode for Youtube. Right-click a Youtube page and choose "Disable force dark page". It it says "Force dark page" instead, then you're not forcing Opera's dark mode on the page and this is not the problem.

2. Youtube's Ambient Mode feature. Click the gear on the video toolbar and turn it off.

more specific: Opera GX also allows you to force pages into dark mode by applying its own dark theme to pages. To control this, there's a global "Force Dark Pages" option at the URL opera://settings/opera-gx. But, you can also override the global on a per-site basis by right-clicking a page and choosing "Force Dark Page" or "Disable force dark page". And, to get a view of all sites that are overriding the global, you can goto the URL opera://settings/content/forceDarkMode. Note though that forcing a page to be dark when a page is already displaying as dark because of its own theme and or an extension you have installed can cause page colors to look weird. So, if you have problems with colors, always check your dark mode extensions, the page's theme, and Opera's dark mode settings to make sure you're only using one method of darkness.