Xioami POCO F1 (called POCOPHONE F1 globally) which launched in 2018 in India has been kept on the table with frequent OTA updates. The MIUI 10.3.5.0 update that was rolled out for the POCO F1 last week, brought May security patch along with a host of improvements and fixes.
According to the user who posted the info on Reddit and MIUI community, the Dark Mode that was received on his device with MIUI 10.3.5.0 is something unexpected and more to the surprise, he couldn’t find any option to disable/toggle the feature.
The user who says he is from Sri Lanka also reported that the system-wide dark mode is configured to all MIUI apps, Phone and messaging apps.

Another user said that,

Interestingly, another user from Indonesia also reported that he too received the dark mode after updating to MIUI 10.3.5.0 and expressed his inability to find any option to disable it.

We expect that Xiaomi might issue an update to fix this soon. Android Q OS update is around the corner, and we fear Xiaomi would bring more issues and bugs which make Reddit and forums busy with users posting their experiences. POCO F1 undoubtedly is a very good device for developers, and these issues will not stop them from updating to next Android version or buy the upcoming POCO F2.