The Google Phone app adds some small but notable changes to the dark mode settings.
In the past, the Google Phone app had a special dark mode switch in the app settings. The switch was of course adjusted according to its subject settings systemic your. Now - found by team of Android Police - The Google Phone app theme settings have been replaced by a floating options tab that includes "Light", "Dark" and "System default".
It's not the most significant change ever made to a Google application, but it lets you define the theme based on your preferences much easier than the previous on-or-off switch. So you now have three options with "System default" being the default option when your application has been updated.
We see the option on several devices that have been upgraded to Android 11, but Android Police notes that the option is available on Appliances using Google Phone which is older than version 51. This indicates that this is part of a switch server and not from the version you are using on device you.
Considering that the Google Phone app acquired the dark mode function in 2018, this is a useful change that will give you more options.