I updated my phone to iOS 13 but my dark mode won’t show

Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support

What about Control Center? How old is your iPhone? What model?

Adjust the iPhone screen brightness and color - Apple Support

After all this back-and-forth, you still have said noting about Control Center. Why?

Use and customize Control Center on your iPhone, iPad, and iPod touch - Apple Support

Might you have set up some Accessibility parameter that's interfering with your display parameters?

About the vision accessibility features on your iPhone, iPad, or iPod touch - Apple Support

As you probably know, the iPhone 6S is the oldest iPhone that supports iOS 13.

I don't know if it has the hardware to support Dark Mode.

But I suggest you try Control Center, as I stated above.

So what is inside Settings->Display & Brightness option.

Check Settings > General > Software Update

Are you certain you're using iOS 13?