Slack Themes
slackthemes.net
slackthemes.net
It's actually influenced my thoughts on how I want to structure one of my own projects. The actually useful free version that as you start using more of the advanced features (but not specific ones) starts costing seems like a good model to get people in and using your product while also incentivizing users to become customers.
It's funny how the color-schemes of well-known UIs are so embedded in our memory!
Other than that, though, good work. :)