Sharing / Like Blocks: compatibility with theme styles

[jeherve]

Splitting off discussion from #36382:

The buttons should also be more defensive about theme CSS leaking in (or allow some CSS in, but in the same way for both buttons, whichever works better). I think the like is an iframe so being more defensive might be better. Now they end up looking different size, and different hover effects depending on theme:

Shadow dom might also be helpful here.

I'm not sure what's the best way to go about this. On one hand, the design should be consistent between the 2 buttons. On the other, wouldn't we want the fonts and styles to match the rest of the site?