To get it up and running, you’ll need the beta app (I’m on app version 126.96.36.199.120). So long as there is room, you can join the beta by following the Google Play link below from your mobile phone.
The app follows your system’s theme, so whether you’re on a Pixel, Samsung, or something else, as long as your system’s theme is set to dark, the app should follow that. There doesn’t appear to be a manual toggle, so if you aren’t a fan of the darker look, you’ll need to disable Night/Dark mode on your device.
With it showing up in the beta, that means it should head out to all users soon.