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Dark mode is available only for a few devices at the moment. It should be available in POCO in the future, but no ETA for that. Also, dark mode is an Android Q feature, so it will be available anyways.
For now, if you want dark mode, you can install Google Messages app and enable dark mode from its settings. This will enable system wide dark mode, but note that some UI elements may not be properly themed. Alternatively, you can use a dark/black theme or flash EU ROM for dark mode.