This tool writes a dark theme to the Slack desktop application. Dark theme was taken from the generated Dark Reader theme that was applied when I visited the web version of slack, like so:

Array.from(document.querySelectorAll('.darkreader')).map((n) => n.textContent).join('\n');

Install python

You really should take the css file from this site, and host it somewhere you control. It's not a great idea to have a script inject an arbitrary CSS file from a domain outside of your control, into an application like Slack that could have sensitive data.

Don't trust me - fork the repo.

Unix

sudo python makeitdark.py

sudo python3 makeitdark.py

Windows

python makeitdark.py

Add this sidebar theme for consistency which kinda makes it look like Mojave dark mode, from slackthemes.net:

#333336,#2e2e31,#666668,#ffffff,#277df6,#d7d5d4,#277df6,#277df6

If you want to uninstall the dark Slack theme you can run with the makeitlight option:

makeitdark.py makeitlight

When Slack updates it will overwrite the installed dark theme. When this happens just re-run the tool to make it dark again.