This repository is inspired from slack-black-theme and especially this issue

• The testing has been done only on Mac as of this writing. Find Slack.app under applications and use Show Package Contents
• Find the file the ssb-interop.js in Slack.app/Contents/Resources/app.asar.unpacked/src/static
• Add the content from code.js at the end of the file and save it
• On restarting Slack, the new theme should be applied

There's a lot of scope for improving the closeness of the theme to Monokai.

• Open Slack with developer menu with this export SLACK_DEVELOPER_MENU=true; open -a /Applications/Slack.app
• Click on Toggle Webapp Devtools under View > Developer
• You can now just use inpect element to get to the html code for a block in question and tweak the css
• Add the css changes in ssb-interop.js and reload Slack View > Reload for the changes to take effect