Works exactly as I'd hoped. For other users: give it a moment to to make the change (I wasn't patient the first time and refreshed the page, which also worked). Thank you, BigsyBiggins, for this plugin!

Great extension, it does just what is needed. Slack should take note of this and add it to there web and desktop apps, it's bizarre that an app which is so dev friendly in other ways doesn't have built in dark mode.

Thanks @BigsyBiggins you're doing the lords work here.