How to dark theme Slack v3.x and v4.0.1 (and not just the sidebar)
Update as of 8.15.2019: Added some styles for the sidebar comments.
Slack's themes are tired (as of this writing) and it's hard to find snippets around the web that really work without having to do some crazy-long setTimeout() or re-do all your work on each startup of Slack.
And yes, there are themes, but actually getting them into Slack has been the big challenge. (Because Slack devs update and break old methods.)
Enough with that. (Note to Slack devs: please keep updating Slack but don't keep breaking our hacky theming mods. Thanks!)