why can't I use dark theme on my Iphone 5s?

Unfortunately iPhone 5s only supports up to iOS 12.5. Dark mode was released with iOS 13. You would need to upgrade to a newer iPhone that can use iOS 13 or greater in order to use dark mode.

moazzam273 Said:

"White theme is not even safe for your eyes!"

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Provide Apple Feedback:

Do your Part: Provide Apple with feedback on this. Apple wont get back to you directly, but the more they hear of this, the more they will know what their customers prefer, and what to include in updates to come.

To Provide Feedback:

1. Go Here: Feedback - iPhone - Apple
2. Select: "Feature Request" for the "Feedback Type"
3. Comment: on why this is not safe for your eyes
4. Proceed form there as necessary

moazzam273

"why can't I use dark theme on my Iphone 5s?: White theme is not even safe for your eyes!"

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Enable Dark Mode:

I think this is the link you need: Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support

Maybe because Apple did not provide the Dark Mode option until iOS 13? ... and your iPhone 5s cannot be upgraded past iOS 12.5.4. However, if you do an Internet search you will find some articles on a dark mode work-around for iOS 12.