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!
1#DCDCDC,#ED6970,#ED6970,#FFFFFF,#FFFFFF,#291d1f,#1c7c5a,#3a4a8ff 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.