Slack now has themes for the sidebar. If you work with multiple teams (like I do), it's a great visual cue to avoid accidental posts to the wrong one.
I particularly like the clever implementation because it's kind of nerdy.1
Open your user preferences in the web app:
Then bring up the sidebar theme and activate the advanced mode. Paste in a string of color codes and boom, an entirely new sidebar theme across all of your devices.
Slack has always been kind of nerdy. Paste color codes into a chat and you automatically see the color swatches in the web app. ↩