Better styling for UI links in book module

Description of the need

Thanks to @herbdool, we just merged some big improvements to the book module into 1.22 (https://github.com/backdrop/backdrop-issues/issues/5331). However, one possible follow-up issue is to improve the appearance of some of the admin links provided by the book module.

I don't think the presentation is perfect, the links feel like they're floating without any styling, both on the node page and in the vertical tab... but I think it's important that they have been added to help management of books. I get the impression further changes are still in mind, so I'm in favor of moving this in and continuing in additional issues. https://github.com/backdrop/backdrop-issues/issues/5331#issuecomment-111...

Proposed solution

No proposals yet. Looking for good ideas.

Alternatives that have been considered

None yet.