Update 2021-11-29: Google Chat has now a native dark mode.
This post and repository idea are osbolete.
I’ve using Rambox for the past year to group my IMs in a single app and it worked pretty well, except that I lost Darkreader’s themes inside the frame.
I found a way to implement custom styles per service. A very, very basic dark mode overlay has been implemented for Google Chat on my GitHub repo: https://github.com/Germain-Italic/rambox-dark-mode
It’s obviously ugly as fuck and it’s not even so dark, but it is. Been using it for 3 days, and 2 nights, and I my skin stopped tanning since then.
I wish I could find some time to implement Darkreader’s API anytime soon.
Meanwhile, feel free to fork and submit PRs.