Microsoft has updated the Feedback Hub for Windows 10 on PC and Mobile for Insiders on the Fast ring. The new version includes a new dark theme setting for the app.
The new version number for the app is 1.1608.2441.0. As you can see from the screenshots above, the new dark theme can be switched on from the Settings menu, along with its light theme. The app also allows for it to switch to the system theme automatically.