google-calendar-theme-sync.user.js

// ==UserScript==
// @name         Theme Color Sync for Google Calendar PWA
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Changes the window title to be the same color as the app background. Mostly useful for Dark Reader.
// @author       eth-p
// @match        https://calendar.google.com/*
// @icon         https://calendar.google.com/googlecalendar/images/favicons_2020q4/calendar_31_256.ico
// @updateURL    https://gist.github.com/eth-p/84609809239f320eb634d3e70f757ad1/raw/google-calendar-theme-sync.user.js
// @downloadURL  https://gist.github.com/eth-p/84609809239f320eb634d3e70f757ad1/raw/google-calendar-theme-sync.user.js
// @grant        none
// @run-at       document-start
// ==/UserScript==

(function() {
    'use strict';

    // Get the theme-color meta element.
    let themeColorMeta = document.querySelector('meta[name="theme-color"]');
    if (themeColorMeta == null) {
        themeColorMeta = document.createElement("meta");
        themeColorMeta.setAttribute("name", "theme-color");
        document.head.appendChild(themeColorMeta);
    }

    function updateTheme() {
        const computedStyles = window.getComputedStyle(document.body);
        themeColorMeta.setAttribute("content", computedStyles.backgroundColor);
    }

    // Update the theme whenever the style changes.
    window.addEventListener('load', updateTheme);
    window.addEventListener('DOMContentLoaded', () => {
        const observer = new MutationObserver(updateTheme);
        observer.observe(document.body, { attributes: true, attributeFilter: ['class', 'style'] });
    });

})();