In the browser version of Slack the favicon shows up white if any message appears. This disables that, keeping the red new-message icon intact
You’ll need a browser blocker to do this (I highly recommend uBlock Origin but any that support the Adblock-style personal blocking lists will work)
For a specific Slack server
In your personal filter list, add the following URL - replacing INSTANCE with your Slack server’s name
For all Slack servers
Same thing but in your personal filter add the URL
Now when a message that doesn’t need your attention comes in you don’t get the distracting white favicon, but will still get the red version of the icon to indicate you’ve got a new direct message. Noice.