A flat dark color scheme based off the base16-tomorrow dark color palette. Someday I'll add some more coloration here, but splitting up the selectors will be a lot of work.

Set your Sidebar Theme to this, under Preferences:
#282A2E,#282A2E,#373B41,#F0C674,#373B41,#E0E0E0,#B5BD68,#CC6666

Thanks to Chris Kempson for the base16 color palettes (http://chriskempson.github.io/base16/).

Big thanks to lacour@userstyles, Jonathan la Cour, for creating the original Slack Night Mode, http://freestyler.ws/style/96498/slack-night-mode, off which this is functionally based (just the colors changed).

Changelog

1.0 - Initially created.
1.1 - Updated for the newer version of slack with some basic cosmetic fixes. (2015/9/18)
1.2 - Updated for more breaking changes. Unreadable usernames and white backgrounds are gone. (2015/11/13)
1.3 - Updated again for another breaking hover-white change. (2015/11/19)
1.4 - Updated for new unread-on-date thing. (2015/11/30)
1.5 - Updated for new channel switcher changes, topic stuff, fullscreen modal, and a few other things. (2016/01/29)
2.0 - Big update based on the new slack-night-mode code, which allows theming, making updates to the core code easier. As a result, there have been a few notable changes to the style.
2.1 - Updated for recent changes.
2.2 - Updated for recent changes. (2016/6/28)
2.3 - Updated for recent changes. (2016/9/3)
2.4 - Updated for recent changes. (2016/9/16)
2.5 - Updated for recent changes. (2016/9/23)
2.6 - Updated for recent changes. (2016/9/30)
2.7 - Updated. (2017/1/26)
2.8 - Updated. (2017/4/12)

Licensed CC0. 2017. Dan Arnold.