As per Google’s new design language (material UI 2.0), things are getting rounded. Earlier Google updated chrome, fit, search, keep and now they have updated the Android Phone app. The new Google Phone app has 3 columns and each contact picture has round design in the favorites tab. The app does retain the dark mode and it looks neat.
On Android Pie, the Google Phone theme depends on phones settings. If the night color scheme is turned on from display settings, the dialer turns dark as well. It is also possible to turn the dialer phone theme to dark from dialer settings. As reported earlier, Google is definitely trying to incorporate the system-wide dark theme in Android. This will reduce the eye strain in dark and save battery on phones having AMOLED displays.
You can grab the Google Phone beta apk from APKMirror, if you want to have it on your phone right now. The app is completely safe to install and there is no risk involved. The official rollout may take time depending on the device and region so you have to wait if you do not want to sideload the app.