Unable to change theme mode on iPhone 16

1. Is this your corporate/organization/enterprise/School Management iPhone? Then please contact the system administrator of your corporate//organisation/enterprise/School Management
2. If it is your device, in that case, do you have any profile installed by your corporate/organization/enterprise/School Management?
   1. Erase the profile added by you. If you have installed a profile on your own, --> Install or remove configuration profiles on iPhone - Apple ...
   2. Please contact the system administrator of the organization who provided the profile to you

Hey there

Try force restart your iPhone :

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button.
4. When the Apple logo appears, release the side button.

Let us know if you still need help!

Hi, maybe if you provide a few more specifics on what you’re trying to do? The only mode I can think of is Light Mode and Dark Mode? Is that what you’re referring to?

You change modes by tapping the desired mode/look you want your iPhone to display.

Adjust iPhone display and text settings - Apple Support

If this isn’t what you want to do, please provide more details.