The buttons in a bubble look like pencil art, and therefore it's not clear they're interactive. I can't tell whether this is intentional or by design.
When I hover my mouse cursor over the "X" close button, it turns grey and beveled (i.e. button like), which helps me understand I can click it. But when I hover the mouse over a "Dismiss" or "Show" button, they don't act the same, so it's confusing visually. I would expect those buttons also to change appearance when I hover over them.
The black appearance does get my attention a bit faster. I'm not sure it's visually pleasing but I guess the idea should be that if you're going to bother putting a notification up, it should really get my attention, and you'll promise to do that as little as possible unless I ask you to do otherwise.
I think you are right that all buttons should have the same prelighting behaviour.
Created attachment 362837 [details]
Here is a minimal patch to make the action buttons prelight in the same way as the close button.
Built in rawhide