Samsung Pay is getting a much-requested feature in latest update
For the time being, SamMobile reports the new dark mode feature is available in India. If you're using Samsung Pay in this country, you won't have to do anything to switch between light and dark themes.
Samsung implemented the dark mode in such a way that the Pay app will change the theme according to your system's settings. The only way to get the new feature is to download the latest update released by Samsung, but it might not be available outside of India for the moment.
In any case, Samsung Pay users can check to see if a new update is available for download by opening the app and heading to Settings / About Samsung Pay. Don't bother to check for the update if Samsung Pay is not supported in your country at all.