A tampermonkey script to enable dark mode even on local files.

dark-pdf.js

// ==UserScript==
// @name         PDF Dark Mode
// @namespace    http://tampermonkey.net/
// @version      1
// @description  A tampermonkey script to enable dark mode even on local files.
// @author       Hirbod Behnam
// @match        *://*/*.pdf
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    // Source: https://superuser.com/a/1527417/940438
    var cover = document.createElement("div");
    let css = `
    position: fixed;
    pointer-events: none;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100vh;
    background-color: white;
    mix-blend-mode: difference;
    z-index: 1;
`
    cover.setAttribute("style", css);
    document.body.appendChild(cover);
})();