Theming Slack (ver 3.3.8)

slack-append-custom-css.js

document.addEventListener("DOMContentLoaded", function() {
  let webviews = document.querySelectorAll(".TeamView webview");

  const cssPath = 'https://raw.githubusercontent.com/Nockiro/slack-black-theme/master/custom.css';
  let cssPromise = fetch(cssPath).then(response => response.text());

  let customCustomCSS = `
  :root {
    --primary: #ccc;
    --text: #0f0;
    --background: #000;
    --background-elevated: #111;
  }
  `;

  cssPromise.then(css => {
    let s = document.createElement('style');
    s.type = 'text/css';
    s.innerHTML = css + customCustomCSS;
    document.head.appendChild(s);
  });

  webviews.forEach(webview => {
    webview.addEventListener('ipc-message', message => {
      if (message.channel == 'didFinishLoading') {
        cssPromise.then(css => {
          let script = `
               let s = document.createElement('style');
               s.type = 'text/css';
               s.id = 'slack-custom-css';
               s.innerHTML = \`${css + customCustomCSS}\`;
               document.head.appendChild(s);
               `;
          webview.executeJavaScript(script);
        });
      }
    });
  });
});

slack-dark.rb

#!/usr/bin/env ruby

require 'active_support'
require 'active_support/core_ext'
require 'thor'

# Usage:
# ./slack-dark.rb
# ./slack-dark.rb [NEW_INDEX_FILE]
# ./slack-dark.rb revert_backup
#
# ruby slack-dark.rb # may work as an alternative, depending on permissions

class SlackDark < Thor
  SLACK_ORIG_INDEX_DIR = '/Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/'.freeze
  SLACK_ORIG_INDEX = File.join(SLACK_ORIG_INDEX_DIR, 'index.js').freeze
  SLACK_ORIG_INTEROP = File.join(SLACK_ORIG_INDEX_DIR, 'ssb-interop.js').freeze
  SLACK_DEFAULT_OVERRIDE_INDEX = File.join('.', 'slack-append-custom-css.js').freeze

  desc 'append [INDEX.js OVERRIDE LOCATION]', "appends custom css script to Slack's index.js and ssb-interop.js"
  def append(file_to_append = SLACK_DEFAULT_OVERRIDE_INDEX)
    system("cp #{SLACK_ORIG_INDEX} #{SLACK_ORIG_INDEX}.bak")
    system("cp #{SLACK_ORIG_INTEROP} #{SLACK_ORIG_INTEROP}.bak")
    puts "Backed up to index.js.bak and ssb-interop.js.bak"

    system("echo `cat #{file_to_append}` >> #{SLACK_ORIG_INDEX}")
    puts "Appended to index.js"

    system("echo `cat #{file_to_append}` >> #{SLACK_ORIG_INTEROP}")
    puts "Appended to ssb-interop.js"

    puts "Done, relaunch Slack!"
  end
  default_task :append

  desc 'revert_backup', "reverts any backed up index.js and ssb-interop.js"
  def revert_backup
    system("cp #{SLACK_ORIG_INDEX}.bak #{SLACK_ORIG_INDEX}")
    system("cp #{SLACK_ORIG_INTEROP}.bak #{SLACK_ORIG_INTEROP}")
    puts "Reverted backedup up index.js and ssb-interop.js"
  end
end

SlackDark.start(ARGV)