How can I change my iPhone theme?

There's no theme on iPhone.

You can change the Wallpaper.

Settings > Wallpaper > Choose a new Wallpaper here

How do you know that? I just see a picture of a device in a charging stand. Nothing about it convinces me it’s in a store. And there is no ”Touch ID“ app in the App Store, but there sure are serveral available for jailbroken devices. There are also theme managers for hacked devices that will, for example, change the battery icon fill color, as shown in that picture (Note that “invert colors” won’t make that change to purple).

So even if in a store I don’t believe that it is an Apple Authorized seller’s store, nor that the device shown is running stock iOS.

That is a photo of a device in a store.

That looks like the Invert Colors setting is on. Settings>General>Accessibility>Display Accommodations...

Though the presence of the "Touch ID" app on the home screen makes me wonder if it's even a real iPhone... There is no "Touch ID app".

The metal clips securing it to the table makes it look like a store device. I did not mean to respond to your response. I meant to respond to the OP. Many apologies. The OP could have liked that setting and taken a photo of it to create the post.

It makes me think it’s jailbroken. There are numerous such apps for hacked iOS devices, although not sure I‘ve heard of that specific one (bioprotect is one with a very similar icon).