Google has made a small adjustment to how its Phone app handles light and dark theme settings. Previously, Phone had its own binary toggle for light and dark theme, which was separately (and non-intuitively) overridden by the system setting. To make things a little bit easier, the binary on/off toggle has been replaced by three separate options: light theme, dark theme, and a system default setting.
Left: Old binary setting. Middle and right: New three-option setting.
The change is pretty simple. Previously, the Phone app had a simple toggle to enable or disable dark theme, which was overridden and grayed-out if you had it enabled system-wide. While that was functional, it meant you couldn't force a light mode while other apps followed the system setting, and it wasn't entirely intuitive. The new three-option setting lets you enable a theme that contradicts the system default setting (and in our testing, it works), or explicitly allow the app to follow that setting — all much simpler.
We're almost certain that the change occurred server-side, or some other component outside the app controls/triggers it. Testing across a variety of devices with different versions, we've had the new options appear on phones running versions of the app as old as v51, while some devices running later releases like v53 didn't have it initially, though it appeared after a reboot. It also doesn't seem to be limited to the beta releases of the app in our testing.
We also don't know precisely when this change rolled out. It could still be in testing, though it appears on all devices we have tested. It may be rolling out widely.
This is the first "major" change to the dark theme setting in the Phone app since it first rolled out a back in 2018, and similar to setting the Play Store and Google Docs suite got earlier this year. It's also logically consistent with the settings change that Gboard received, and I anticipate we'll see similar tweaks land for other Google apps that support manual settings for dark theme.