Is there dark mode on an iPhone 8 Plus

Here is an article from Apple --> Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support, this has a link --> Learn how to use Control Center on your iPhone or iPod touch, or on your iPad.

Dark Mode was introduced with iOS 13 and yes, it works on your iPhone 8 (if you update it, since from your products list it seems to run the prehistoric iOS 12).

Regards

Giulio

I have iPhone 8 iOS 15.6.1 which has Dark Mode.

So iPhone 8+ will surely have.