Currently, this dark mode feature for Google Play store is available on selected device but with time it will be launched for more people out there. Google has announced previously that Android 10 will come with a whole system integrated with dark mode so that the overall interface is effective on eyes. Furthermore, Android 10 will also incorporate this dark mode on third-party apps.
Google Play store Dark Mode Rolls out for Pixel Devices
Android 10 has already launched for Pixel devices, the Google Play Store should be greatly benefitted from this. This mode on Google Play Store would be quite different as it will be kind of grey. Pixel 2XL users have already received it, so believe in a short period of time, many other users will be getting this feature as Google plans to expand this theme to other phones as well.