After trying multiple solutions involving unpacking asar files and injecting css, only to need to fix it every few days after slack updates, I think I've finally got a solution that will require minimal effort to stay functional. Note that this involves using slack in a Chrome window instead of the desktop app.

1. Install a dark mode extension in Chrome (I use this one)
2. Follow these steps to create a shortcut to Slack that opens in a new window without tabs or address bar

I didn't want to use slack in chrome before because of the address bar/tabs/bookmarks cluttering up the space, but with the minimal window it looks just as good as the native app (and runs just as well in my experience).

Anyways, hopefully someone finds this useful. If this has been posted before, just let me know and I'll remove it (I didn't see it, but who knows considering how bad reddit's built-in search function is).