Slideshow 'Add slide' button not visible in dark mode

If you are using dark mode in a theme then the 'upload an image' button is not visible.

Example here:
http://g.recordit.co/JvZFk8BOzg.gif

I fixed this in my theme by adding:

```
.wp-block-jetpack-slideshow__add-item-button {
background: rgba(255, 255, 255, 0.5);
}

.wp-block-jetpack-slideshow__add-item-button:hover {
background: rgba(255, 255, 255, 0.2);
}
```

Info on dark mode here:
https://developer.wordpress.org/block-editor/developers/themes/theme-support/#dark-backgrounds

Locate the Jetpack slideshow styles for .wp-block-jetpack-slideshow__add-item-button and reproduce the issue in a WordPress theme using dark mode. Compare the button and hover states with the reported CSS and verify that the “upload an image” control is visible in both states.

Written by the indexing model from the issue text.