A Slack sidebar theme generator for themer.
Install this module wherever you have themer installed:
npm install themer-slack
Then pass themer-slack as a -t (--template) arg to themer:
themer -c my-colors.js -t themer-slack -o gen
themer-slack will generate a themer-slack-dark.txt or themer-slack-light.txt file (or both), depending on the color set you passed to themer. Simply copy the contents of this file and paste into the custom theme input in Slack's preferences.