I think it must be android version. I can't think of any other things. I think people who got dark theme are those people who have Android 10 or more. If it's true, it's still unfair, because on my Android 9, I received dark them for Google play store, Google calendar, Google messages, Google calculator, Google Photos, Google Keep, Google contacts, almost all my Google products finally received dark theme. The only application that didn't receive it is Gmail. Why no Gmail? Why?! So, unfair
Unfortunately, I can't upgrade to Android 10. One android upgrade for Redmi series is the Xiaomi policy and Android 9 was the upgrade of Android 8 on my phone.