jsfiddle_userscript.js

// ==UserScript==
// @name         jsfiddle auto dark switch
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @include        https://jsfiddle.net/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const e = document.querySelector("#editor-theme");

    const isDark = window.matchMedia('(prefers-color-scheme: dark)');
    if(isDark.matches) {
        e.href = e.href.replace(/editor-.*?.css/, "editor-dark.css");
        document.head.insertAdjacentHTML('beforeend', '<link href="https://jsfiddle.net/css/dist-skeleton-dark.css?update_06_08_2019_1" id="skeleton-theme" rel="stylesheet" type="text/css">');
    } else {
        e.href = e.href.replace(/editor-.*?.css/, "editor-light.css");
        document.head.insertAdjacentHTML('beforeend', '<link href="https://jsfiddle.net/css/dist-skeleton-light.css?update_06_08_2019_1" id="skeleton-theme" rel="stylesheet" type="text/css">');
    }

    const s = document.querySelector("#skeleton-theme");

    isDark.addListener((evt) => {

    if(evt.matches && evt.media.includes('dark')){
            if (e) e.href = e.href.replace(/editor-.*?.css/, "editor-dark.css");
            if (s) s.href = s.href.replace(/skeleton-.*?.css/, "skeleton-dark.css");
        } else {
            if (e) e.href = e.href.replace(/editor-.*?.css/, "editor-light.css");
            if (s) s.href = s.href.replace(/skeleton-.*?.css/, "skeleton-light.css");
        }
    });
})();