Soften edges / shading around admonitions

[choldgraf]

Description

In the book theme we recently softened the admonitions a bit, since a lot of web design best-practices recommend rounded corners rather than hard edges, and we tweaked the shadowing as well.

Would others be interested in upstreaming these improvements to this theme? It would look something like this:

OLD:

NEW:

Implementation details

Here's the PR where this was added in the book theme, it wasn't too much CSS! https://github.com/executablebooks/sphinx-book-theme/pull/352/files

[jorisvandenbossche]

Sounds good to me! I would maybe use a slightly smaller border-radius (you used 0.4em in sphinx-book-theme, eg furo uses 0.2rem)