Slideshow block: Controls have no contrast (on select themes)

### Impacted plugin

Jetpack

### Steps to Reproduce

1. On a Jetpack-connected site install and activate the Twenty Twenty One theme.
2. Add a Slideshow block to a new post, and add more than one image.
3. Save the draft and publish or view the preview - clicking the controls will result in a white square (no control indication).

### A clear and concise description of what you expected to happen.

The controls should show contrast.

Reported internally here: p8oabR-Uj-p2#comment-6507

### What actually happened

The controls do not show any contrast - just a white square.

### Browser

Google Chrome/Chromium, Mozilla Firefox

### Other information

This is not a recent issue, testing back to 11.0 it was still the case then. Also, so far it's only been reproduced in one theme (Twenty Twenty One) - however that one theme has more than a million active installations.

### Platform (Simple, Atomic, or both?)

Self-hosted

### Reproducibility

Consistent

### Severity

Some (< 50%)

### Available workarounds?

_No response_

### Workaround details

_No response_

Reproduce the Jetpack Slideshow block on a self-hosted site using the Twenty Twenty One theme, with multiple images, in Chrome or Firefox. Inspect the Slideshow block's control styling and compare it with a theme where the controls are visible; done means the controls have clear contrast and their indication is visible when clicked.

Written by the indexing model from the issue text.