[Feature] Presskit SVGs doesn't work well with non-white backgrounds

## Problem
Presskit SVGs don't work well with non-white backgrounds.

Current behavior on the left and desired behavior on the right.

## Description
The SVGs used by the HTML produced by the ["License chooser"][1] have a solid white background inside the circle.
The icons are hosted here and are part of the "Presskit" e.g.:
https://mirrors.creativecommons.org/presskit/icons/by.svg

The white background icons from "Presskit" are inconsistent with the icons here (which are transparent):
https://github.com/creativecommons/vocabulary/tree/main/src/svg/cc/icons

I suggest having an extra set of icons in the "Presskit" with transparent centers.
It is probably a good idea to leave the current ones as is for backwards compatibility.
Maybe even use these in the "License chooser" by default moving forward.

## Alternatives
Leave everything "as is", but having icons with transparent backgrounds available, we get a web that looks better 😅

## Additional context
Maybe also provide SVGs with white "fill color" for dark (themed) websites.

## Implementation
- [x] I would be interested in implementing this feature. (but I would need some pointers as to where I can find this "Presskit")

[1]: https://chooser-beta.creativecommons.org/