UserScript: Always keep YouTube header in dark mode

YouTube-dark-header.user.js

// ==UserScript==
// @name         Always keep YouTube header in dark mode
// @namespace    https://nileane.fr/
// @version      1.2
// @description  Always apply 'dark' + 'theater' + 'is-watch-page' properties, and set 'frosted-glass-mode' property to 'none' on YouTube header
// @author       Niléane
// @match        *://www.youtube.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function applyAttributes() {
        const masthead = document.querySelector('ytd-masthead');
        if (masthead) {
            masthead.setAttribute('dark', '');
            masthead.setAttribute('theater', '');
            masthead.setAttribute('is-watch-page', '');
            masthead.setAttribute('frosted-glass-mode', 'none');
        }
    }

    applyAttributes();

    const observer = new MutationObserver(applyAttributes);
    observer.observe(document.body, { childList: true, subtree: true });
})();