Contains gruvbox (dark) colors, wallpapers, etc.
https://github.com/morhetz/gruvbox
Copy and paste this into a Slack DM to yourself, and then click the "Switch sidebar theme" that appears, or go into Preferences -> Themes -> Colors and paste.
#282828,#3c3836,#ebdbb2,#1d2021,#3e313c,#ebdbb2,#689d6a,#fb4934
https://chrome.google.com/webstore/detail/gruvbox-theme/ihennfdbghdiflogeancnalflhgmanop?hl=en-GB