GitHub Gist apply dark theme on secret Gists

github-gist-apply-dark-theme-on-secret-gists.user.js

// ==UserScript==
// @name         GitHub Gist apply dark theme on secret Gists
// @namespace    https://github.com/benok/
// @description  Apply dark theme to your secret Gists on GitHub Gist (Choosing "Sync with system" from Theme prefs is required!)
// @include      https://gist.github.com/*
// @run-at       document-start
// @version      2025.4.11.3
// @homepage     https://gist.github.com/benok/69b98fc38aa5884234fe50326ef014ae
// @downloadURL  https://gist.github.com/benok/69b98fc38aa5884234fe50326ef014ae/raw/github-gist-apply-dark-theme-on-secret-gists.user.js
// @author       benok
// @grant        none
// @license      MIT
// ==/UserScript==

let apply_theme = function()
{
    if ( (document.querySelector('span.Label.v-align-middle')||
           document.querySelector('span.Label.Label--secondary')/* Editing page */).textContent.includes('Secret')) {
         console.log('secret gist. set data-color-mode to dark');
         document.querySelector('html').setAttribute('data-color-mode', 'dark');
    } else {
         console.log('public gist. set data-color-mode to light');
         document.querySelector('html').setAttribute('data-color-mode', 'light');
    }
};

let make_private_gists_bg_gray = function ()
{
    console.log('make_private_gists_bg_gray()');

    let lbs = document.querySelectorAll("span.Label");

    for (const lb of lbs) {
        if (lb.textContent.includes('Secret')) {
            lb.style.cssText += 'background-color:#434343; color:#ffffff;'
            lb.closest('.gist-snippet').style.cssText += 'background:#cfcfcf;';
        }
    }
}

function hook_body_attr_change(hook) {
    // https://stackoverflow.com/a/11546242/26736
    let MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

    var observer = new MutationObserver(function(mutations, observer) {
        hook();
    });
    observer.observe(document.body, {
        subtree: false,
        attributes: true
    });
}

(function() {
    'use strict';
    hook_body_attr_change(apply_theme);
    window.onload = make_private_gists_bg_gray;
})();

Having trouble now.
Setting html's data-color-mode doesn't update the actual page theme. :-(
(Setting the default theme to dark, it works partially, but unstable. May be needed to change apply timing.)

Please choose "Sync with system" from Theme preferences and get this script working again.