youtube_auto_theme_switcher.user.js

// ==UserScript==
// @name         YouTube auto theme switcher
// @namespace    https://jozen.blue/
// @version      1.0.0
// @description  Switches the YouTube theme cookie based on current time.
// @author       Zennn
// @match        https://www.youtube.com/*
// @exclude      https://www.youtube.com/embed/*
// @icon         https://www.google.com/s2/favicons?domain=youtube.com
// @run-at       document-start
// @grant        GM.cookie
// @noframes
// ==/UserScript==

(function() {
    'use strict';
    const light = '40080000';
    const dark = '40000400';
    let mode = light;

    const today = new Date().getHours();
    // Light mode from 7AM until 8PM
    if (today > 5 && today < 19) {
        mode = light;
    } else {
        mode = dark;
    }

    GM.cookie.list({ name: 'PREF' }).then((cookies) => {
        const { value, expirationDate, httpOnly, secure, domain } = cookies[0];
        const params = new URLSearchParams(value);

        // Stop if already correct theme
        if (params.get('f6') === mode) {
            return;
        }

        params.set('f6', mode);

        GM.cookie.set({
            name: 'PREF',
            value: params.toString(),
            expirationDate,
            httpOnly,
            secure,
            domain
        }).then(() => {
            // Won't even notice this reload cause it happens at document-start
            window.location.reload();
        })
    });
})();