Slideshow Shortcode: individual image backgrounds are black and do not match slideshow background/border

#### Steps to reproduce the issue

1. Create a new post or page and insert a Classic Block
2. Click on the Add Media button. On the left, click "Create Gallery"
3. Select multiple images, preferably with differing aspect ratios. Then click the "Create a new gallery" button
4. Use the "Type" dropdown to select "Slideshow" and then click "Insert Gallery"
5. Publish and view your new post/page

#### What I expected
The slideshow element should be one consistent color, where the background of the image matches the border of the element itself (or more accurately, the `slideshow-slide` element's background should be transparent so the `slideshow-window` element's background is fully visible) as shown below

#### What happened instead
A black background is applied, inline, to the `slideshow-slide` element.

**Screenshots**
Expected result, one consistent background behind the image:

Actual result, a black background with a dark gray border (and the inline styling causing the black background)

Reproduce the issue through a post or page using a Classic Block, the Add Media gallery flow, and the Slideshow type. Start by locating the slideshow shortcode output and inspect the inline background styling on slideshow-slide relative to slideshow-window. Done means individual image backgrounds no longer appear black and match the slideshow background or border.

Written by the indexing model from the issue text.