Sharing / Like Blocks: compatibility with theme styles

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:
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> [Shadow dom](https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_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?

The issue names no implementation files, tests, or entry points, so begin by locating the Sharing and Like button components and their theme-style boundaries. Done means both buttons have consistent sizing and hover behavior across themes, with an explicit decision about whether site fonts and styles should carry through.

Written by the indexing model from the issue text.