Google has been teasing a Dark Mode for its Phone app
for about a month now but didn't release it yet. The changelog of the last Google Phone update mentions that a Dark Mode is on the way, but no timeframe is provided for the release of the feature.
However, it appears that a Dark Mode has just made its way to the beta version of Google Phone
, which suggests a final release might happen very soon.
The option to switch to the dark theme will be added to the Settings / Display Options, but the setting is not yet visible in the stable version of Google Phone.
Many of Google's apps received Dark mode settings, including Android Messages
. More will probably get it in the coming months, but it looks like Google Phone might be the next one. So, here is hoping the new feature will be made available sooner rather than later.