We just published Compass 1.30-beta, which includes Dark Mode in preview! You can find the new beta in our download center.
While in the upcoming months we will be refreshing the general visual look of Compass including a properly designed dark theme, we thought we'd ship the first version of the dark theme now so you can start using it right away.
You can enable the dark theme from the MongoDB Compass Beta system menu on macOS and from the Help system menu on Windows and Linux (we know it's a bit of an odd location and we are planning to move it).
Feel free to reach out if you have any feedback.