Mi Home got dark mode support quite a while ago. But... you could only turn dark its main screen where you can select the device. As soon as you enter the device plugging, it would become light again.
This time, Mi Home got a separate section in settings called Experimental features where you can enable dark mode for plugins. After enabling, you will see device-specific views in dark mode as well. Looks like some parts are going to remain light to differentiate buttons or other UI elements.
This feature should be available in version 6.9.601 Beta.