Find your Slack's application directory.

• Windows: %homepath%\AppData\Local\slack\
• Mac: /Applications/Slack.app/Contents/
• Linux: /usr/lib/slack/ (Debian-based)

Navigate to the latest installed version and then open the following file in your favorite editor resources/app.asar.unpacked/src/static/ssb-interop.js

Add the following code at the end of the file

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'https://raw.githubusercontent.com/ashutosh-shirole/slack-dark-theme/master/dark.css',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

And thats it. Reload slack Ctrl+R and you should get a dark theme.

You will have to do this everytime you install/update your slack.

If above changes dont change sidebar make the following changes

1. go to: Slack Preferences > Sidebar > Theme
2. Clear the textbox for custom theme
3. Paste the following contents in the textbox

#222222,#444A47,#D39B46,#FFFFFF,#434745,#FFFFFF,#99D04A,#DB6668

IMPORTANT NOTE This workaround will request CSS file from this repository. Using remote CSS file can be a risk. Recommended way is to create a local copy of the css file and use that file.

If you find following these steps tedious on every update I have added a shell script enable_slack_dark_mode.sh which can be run on every update of slack. This is only for linux and can be adapted for other OS too. Feel free to adapt/contribute.

For slack developer tools one has to open slack in developer mode. To do that use (for macOs):

export SLACK_DEVELOPER_MENU=true && open /Applications/Slack.app

Adapted from slack-night-mode