These instructions apply to both iPhone and iPad with screenshots depicting the iPhone 11’s screen.
Here’s what to do.
You’ll need to haveSiri enabledon your iPhone or iPad to do these things.

Siri will now respond in the affirmative and switch off Dark Mode for you.
Here’s what to do to switch off Dark Mode in this way.
Swipe down diagonally from the upper-right corner of your iPhone or iPad’s display to open Control Center.

Hold your finger down on the Brightness indicator.
TapDark Mode Onto toggle it to Dark Mode Off.
Tap on a blank part of the screen to return to the Control Center.

Here’s what to do.
TapLightto switch off Dark Mode.
Here’s what to do.

Dark Mode will now automatically activate when the sun sets but otherwise remain switched off throughout the day.
TapOptions > Custom Scheduleto change when Dark Mode is turned on or off.
Why Should I Switch off Dark Mode on iPhone and iPad?
Dark Mode works best during the evenings and nighttime.