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Here’s a project that aims to help solve this issue called Tasty Imitation Keyboard from Alexei Baboulevitch.

Tasty Imitation Keyboard provides a custom keyboard with a similar look and feel to the native iOS keyboard, but can be used as a baseline for creating 3rd party keyboards. The entire keyboard is drawn using Core Graphics, and utilizes auto layout for easy customization.

This image from the readme shows the Tasty Imitation Keyboard in action:

You can find Tasty Imitation Keyboard on Github.