This introduces dark mode styling for the file attachment thumbnails,

helping them fit in on the review request page and addressing the

invisible text issue that would occur.
Along with that, some interaction issues have been addressed. There were

two issues in our styling:


When editing a caption, the caption area and actions area didn't feel

   seamless, as they were semi-transparent and had some overlaps that

   looked strange. These are no longer semi-transparent during edit.


When editing, moving the mouse over another thumbnail made the

   thumbnail being edited semi-transparent, which didn't feel right. It

   now stays at 100% opacity while in edit mode.

Tested the visuals for the file attachments in light and dark mode,

including the caption area, empty captions, actions menu, and the

unsaved caption indicator.
Tested editing the caption and then moving the mouse over another

thumbnail. Verified that the file attachment did not fade out.