Rambox dark panel
Rambox doesn't have a dark theme yet, but you can mimic it easily.
Rambox dark theme
Rambox allows having separate application for messaging services, which actually is a simplified version of Chrome. The drawback here is lack of extensions, namely Stylish and CareYourEyes because some services provide dark theme out of the box, like GMail, but Slack or Telegram doesn't have it. Let's see how this could be improved.