[Bug]: Card hover state wrong when image contains transparency

### What happened?

[not sure if adding issues here still a good idea, lemme know if not]

We will have more of transparent card images, so an old issue resurfaced. It was apparently fixed in https://github.com/guardian/dotcom-rendering/pull/1023 (and confirmed in [here](https://github.com/guardian/frontend/issues/8963#issuecomment-565297174)), so it may be a regression in fairground (?).


Also, it looks like we should style the underline explicitly too: notice how Firefox (left) is nicer than Chrome (right). I can open another issue if easier…

### Where is this happening?

_No response_

### What is the impact?

Impact is aesthetic, minor.