Dark mode not appearing as an option on ipad, how to make it appear?

Solution 1:

iOS 12 doesn't have Dark Mode. It was new to iOS 13.

You can achieve something similar by going to

Settings > General > Accessibility and choose Display Accommodations. Then click Invert Colours.

If you want to go all funhouse crazy, toggle the Classic Invert mode. But to get an inverted iOS that's closer to a true dark mode, you'll want to select Smart Invert. That way iOS 12 actually tries to keep things from going too crazy.

Courtesy of Forbes - iOS 12.4 Public Beta 4 Released, Is Boring, Here's How To Get Dark Mode Now

Your iPad Air ME898LL/A 4,1 unfortunately cannot use iOS 13, it is stuck at 12. Info: Everymac