Option to change lightbox background when viewing zoomed images

### Steps to reproduce

1. Send someone an image with a transparent background (often black-on-transparent)
2. Lightbox shows it as black-on-black, even in light theme.
3. Feel frustrated that there isn't a way to change the lightbox to be a lightbox rather than a blackbox temporarily so you can view the image.

### Outcome

#### What did you expect?

Legible black-on-transparent images.

#### What happened instead?

Illegible ones.

### Operating system

_No response_

### Application version

_No response_

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No