Dracula — A dark theme for Slack and 40+ apps

Go to User Menu > Preferences > Sidebar Theme
In the bottom of the window, look for a "customize your theme and share it with others" link
Copy and paste the values below

#282A36,#44475A,#44475A,#8BE9FD,#6272A4,#FFFFFF,#50FA7B,#FF5555

This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.