The Previous/Next file navigation buttons had a few style issues on dark

mode, some regressions, and bad interactions.
They were bunched up against the review UI, which regressed a while back

when we renamed the review-request CSS class to rb-c-review-request.

That had intended to ensure enough space between the button and the

review UI, but that had broken. That spacing has been restored.
The colors used on the navigation buttons have been updated to work in

both light and dark modes.
Thumbnail backgrounds now use a darker color by default, and a matching

text color. This gives us text-based file attachment thumbnails that

look appropriate on dark mode. For consistency with rendering, we use

the "diff equals" theme color. We may want to revisit that down the road

when we build a proper CSS component for thumbnails, instead picking the

theme color for text-based attachments only.
And finally, an interaction issue with navigation has been fixed. We

were wrapping the entire attachment in an <a href>, but since that's

an inherently inline element and we're putting complex block-level

elements inside of it, the browsers are forced to try to compensate in

some form. They seem to do that these days by wrapping most inner

elements with their own <a href>, creating a mess of styles. The end

result doesn't even make for a fully-clickable attachment.
The solution is the same as what we use for file attachment thumbnails

on a review request. We now have an <a href> overlay that sits on top

of the whole element and consumes all click events, giving us the

ability to navigate but also to respect browser link interactions. The

result is a wider clickable area for navigation. This has also been made

accessible.

Tested previous and next navigation in both light and dark mode, with

different types of file attachments.
Tested the main review request thumbnails for image-based, PDF-based,

and text-based file attachments.