Set-SlackDarkMode.ps1

# Run this script in an elevated prompt, and quit and reload Slack after

$slackBaseDir = "C:\Program Files\Slack\"
$installations = Get-ChildItem $slackBaseDir -Directory | Where-Object { $_.Name.StartsWith("app-") }
$version = $installations | Sort-Object { [version]$_.Name.Substring(4) } | Select-Object -Last 1
Write-Output "Choosing highest present Slack version: $version"

$customContent = @'
// First make sure the wrapper app is loaded
document.addEventListener("DOMContentLoaded", function() {

   // Then get its webviews
   let webviews = document.querySelectorAll(".TeamView webview");

   // Fetch our CSS in parallel ahead of time
   const cssPath = 'https://raw.githubusercontent.com/Nockiro/slack-black-theme/3ea2efdfb96ccc91549837ab237d57104181bbf8/custom.css';
   let cssPromise = fetch(cssPath).then(response => response.text());

   let customCustomCSS = `
   :root {
      /* Modify these to change your theme colors: */
      --primary: #61AFEF;
      --text: #ABB2BF;
      --background: #282C34;
      --background-elevated: #3B4048;
   }
   div.c-message.c-message--light.c-message--hover
	{
	color: #fff !important;
	background-color: #222 !important;
	}

	span.c-message__body,
	a.c-message__sender_link,
	span.c-message_attachment__media_trigger.c-message_attachment__media_trigger--caption,
	div.p-message_pane__foreword__description span
	{
			color: #afafaf !important;
	}

	pre.special_formatting{
		background-color: #222 !important;
		color: #8f8f8f !important;
		border: solid;
		border-width: 1 px !important;

	}
   `

   // Insert a style tag into the wrapper view
   cssPromise.then(css => {
      let s = document.createElement('style');
      s.type = 'text/css';
      s.innerHTML = css + customCustomCSS;
      document.head.appendChild(s);
   });

   // Wait for each webview to load
   webviews.forEach(webview => {
      webview.addEventListener('ipc-message', message => {
         if (message.channel == 'didFinishLoading')
            // Finally add the CSS into the webview
            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);
            })
      });
   });
});
'@

Add-Content "$($version.FullName)\resources\app.asar.unpacked\src\static\index.js" $customContent
Add-Content "$($version.FullName)\resources\app.asar.unpacked\src\static\ssb-interop.js" $customContent

Write-Output "Mod done - please restart Slack"