Solarized was created by Ethan Schoonover and you can find out more about the design work that went into it on his project page. His main git repository has ports to many other editors and consoles.
Solarized Dark was ported for slack by Daniel Stinebaugh and can be found at https://github.com/dstinebaugh/Slack-Solarized-Dark-Theme/.
Just copy and paste the color codes into your custom sidebar theme!
#002b36,#073642,#657b83,#93a1a1,#073642,#839496,#859900,#d33682
Info on where sidebar themes can be found and edited here on slack