Google Pay update brings dark mode to Android users
According to AndroidPolice, a certain version of Google Pay (2.96.264233179) has the new dark theme, but it's only showing up for some users on the Google Play Store. You can download the APK file from the source if you don't want to wait for Google to release it in your country.
If you're lucky to get the updated version of Google Pay, the app will automatically switch to dark mode if the battery saver is enabled, but you can also manually set the app to use dark theme whenever you want.
Once Android 10 gets released, the system-wide dark mode will take over, so it makes sense for Google to want to add this feature to as many of its app as possible until the new OS update hits compatible devices.