Tampermonkey script to sync Feedly theme to OS

feedly-auto-theme-switcher.user.js

// ==UserScript==
// @name         Feedly Auto Theme
// @version      0.1
// @description  Automatically switches between light and dark themes on feedly.com based on system settings.
// @author       13xforever
// @match        https://feedly.com/*
// @run-at       document-idle
// @license MIT
// ==/UserScript==

(function() {
    "use strict";
    if (!window.matchMedia) {
        return;
    }

    const LIGHT = "theme--light";
    const DARK = "theme--dark";
    const switchSelector = 'button[aria-label="Switch Between Themes"]';

    function waitForElm(selector) {
        return new Promise(resolve => {
            if (document.querySelector(selector)) {
                return resolve(document.querySelector(selector));
            }

            const observer = new MutationObserver(() => {
                if (document.querySelector(selector)) {
                    resolve(document.querySelector(selector));
                    observer.disconnect();
                }
            });

            observer.observe(document.body, {
                childList: true,
                subtree: true
            });
        });
    }

    function checkMode(q) {
        try {
            let currentNightMode = document.body.classList.contains(DARK) ? DARK : LIGHT;
            let newNightMode = q.matches ? DARK : LIGHT;
            if (newNightMode !== currentNightMode) {
                document.querySelector(switchSelector).click();
            }
        } catch (ex) {
            console.log(`Failed: ${ex}`);
        }
    }

    waitForElm(switchSelector).then(() => {
        setTimeout(() => {
            const query = window.matchMedia('(prefers-color-scheme: dark)');
            checkMode(query);
            query.addEventListener('change', checkMode);
        }, 200);
    });
})();