The Slack (desktop) client allows you to create your own themes. But did you know that you can also set the theme via a message. Simply send a message with the contents being 8 comma separated hex codes. You don't have to send the message to anybody either - Just send it to yourself!
These stand for the following elements:
- Sidebar & top bar background
- Highlighted channel background
- Highlighted channel text
- Highlighted channel background (hover)
- sidebar text
- Online indicators
- Message indicator
As soon as you send this to yourself a little button will appear giving you the ability to trigger a theme change.