Userscript for making Discord sidebar in Light theme dark again, after September 2019 update.

discord-dark-sidebar.user.js

// ==UserScript==
// @name Discord dark sidebar for Light theme
// @namespace Violentmonkey Scripts
// @match https://discordapp.com/*
// @grant none
// ==/UserScript==

var style = `

  /*
    Invert but keep hue on server lists (guilds) and sidebar.
      - invert(100%): invert colors
      - hue-rotate(180deg): restore the original hue of the inverted colors
  */
  [class*="guilds-"],
  [class*="sidebar-"] {
    filter: invert(100%) hue-rotate(180deg);
  }

  /*
    Try to restore original colors of server icons and user avatars.
    Not perfect, looks like color vibrance is a bit lost somehow, but better than nothing.
  */
  [class*="guilds-"] [class*="blobContainer-"] img,
  [class*="sidebar-"] [class*="avatar-"] img {
    filter: invert(100%) hue-rotate(180deg);
  }

`;

ready = (fn) => {
  if (document.attachEvent ? document.readyState === "complete" : document.readyState !== "loading"){
    fn();
  } else {
    document.addEventListener('DOMContentLoaded', fn);
  }
}

var run = () => {
  ((cssString) => {
    var style = document.createElement('style');
    style.innerHTML = cssString;
    document.getElementsByTagName('head')[0].appendChild(style);
  })(style);
};

ready(run);
run();