If you make this allow for an all black theme (INCLUDING the main chat window) I'll happily use it. I use a black theme (there's a few forks of https://github.com/widget-/slack-black-theme) but Slack breaks them beyond just dumping the code back into the css files constantly.
Glad to hear it! I'm undecided re open source, two reasons: I wouldn't mind trying to sell it and take a chance at the 'indie developer' dream, and also managing a community project i think is beyond me.