Carousel: update colors of the previous / next buttons

### Impacted plugin

Jetpack

### What

Previous / Next buttons do not follow the same patterns as the other icons in the Carousel view. Let's update them to be consistent.

### How

Previous / Next buttons have 2 states: when not hovered, they're faded out, and they only light up when hovered:

https://user-images.githubusercontent.com/426388/187917688-f63dc57f-a401-4103-9172-7c6ab049b0a3.mov

The other buttons in the interface, however, are all white by default. Maybe the prev/next buttons should too?

Start by locating the Jetpack Carousel implementation and the previous/next button styles, then compare them with the other icons in the Carousel view. The issue's video and description show the current faded-on-idle behavior; done means the buttons follow the agreed consistent color treatment.

Written by the indexing model from the issue text.