In the last two months, Google Pay has undergone a huge transformation, in terms of appearance and function. Google merged the international Pay app with its Indian counterpart (formerly called Tez) after the rewriting of the latter Flutter and it even received a new logo recently. But unlike most other Google services, the new GPay has so far lacked a proper dark mode, but that ultimately goes to the beta version of the app.
Google Pay India showed signs of an upcoming dark mode with a trimmed splash screen in September, but the main app interface itself has retained its striking white layout. With the latest beta, GPay has launched a more complete dark mode implementation that works across the board.
The program settings, however, do not have a switch to start the mode manually; it will automatically follow your custom theme settings. Google may introduce a manual option in some future, as many of its other programs have, but it remains to the automatic mode now.
The new dark theme is being developed with GPay beta version 119.1.4, although it is not yet widely available for those enrolled in the beta app through the Play Store. You can instead grab the APK Mirror app to get the new theme sooner.