They have dark themes, but those only let you customize the sidebar colors, leaving the main window white.

With the release of system-wide dark modes on macOS Mojave and Windows 10, Slack feels very out of place.

Its fairly easy to do, but since it will be overwritten every time you update, you will have to do this multiple times.

But the CSS files for Slack are buried in the source, so youll have to load your own themes.

We found there were some issues with the theme on macOS Mojave though, so if it doesnt work then you can try this fork, which says it works on macOS only but may work for Windows users as well.

Make sure Slack is closed, open that file in your favorite text editor, and scroll to the bottom.

This way, you can just drag it into the directory to overwrite the newest version.

The loading screen will still be white, but the main app window will blend in much better with the rest of your system.

If you dont know CSS, or just want to make a minor change, there are four color variables defined before loading the CSS, so you can just edit those with your own colors.

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Themes For Mac Slack App