Custom styles on Previous and Next Rectangle Elements

[SaadBazaz]

It'd be great if we could adjust those as developers. I have a use case where it'd be highly necessary:

The Stories are full-paged but the progress bar remains in the center. Thanks to one of the latest PRs (#216) I was able to have the story background covering the screen. Now what remains is being able to tap anywhere on the first half of the entire screen to go Back, and the second half of the entire screen to go Forward.

(I'm sure more usecases could be thought of but that's what's at the top of my head rn)