Dark Mode is now fully supported by Observatory on macOS 10.14 Mojave. It even works on macOS 10.11–10.13, but there it only affects the appearance of the Library window. In the new Appearance preference pick System to follow the system setting, or choose either Dark or Light to override it.
I think Observatory looks pretty good in Dark Mode.
We’ve also packed a bunch of little changes and bug fixes in there. See the release notes for the complete list.