I can confirm that this add-on is no longer working with the latest Slack webpage. (mac0S 10.14x, iMac, firefox 68.0.1) - side bar disappears when moused over, page flashes periodically) -- Prior to this week, the theme worked just fine.

It stopped working properly about a week ago. Now it affects the sidebar completely, and causes the screen to flash black occasionally.

I wish it didn't touch the sidebar, because it makes my sidebar themes look weird. Besides that, a much needed upgrade to Slack

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.