That’s definitely Jetpack and they’re doing something pretty wrong to cause this issue. Rather than using an actual button and inheriting the theme (or at a minimum the browser’s) styling (which would also inherit modifications made by plugins), they’re making fake button and applying arbitrary styles that need to be defined by every theme.
I think the best solution is to ask the Jetpack devs to fix this issue, which, although apparently intentional, really doesn’t make sense. They’re making it unnecessarily difficult for themes and plugins to have compatibility. If they offer a good reason why they won’t/can’t fix it, then I can consider adding their selector to this plugin, but that shouldn’t be necessary to accomplish what they’re trying to accomplish.