Click on the Raw button below and Tampermonkey should automatically pick it up. Just press install.

no-youtube-ambilight.user.js

// ==UserScript==
// @name         No YT Ambilight
// @namespace    https://oisann.net/
// @version      0.3
// @description  Remove YouTube Cinematics Effect.
// @author       Oisann
// @match        https://www.youtube.com/watch*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    const attribute = "style";
    const style = "display: none";
    function removeAmbilight() {
        const element = document.getElementById('cinematics');
        if (element) {
            if (element.getAttribute(attribute) !== style) {
                element.setAttribute(attribute, style);
            }
        }
        window.requestAnimationFrame(removeAmbilight);
    }
    window.requestAnimationFrame(removeAmbilight);
})();