We had multiple places were we were styling comment thumbnails. Both

reviews and the review dialog had their own variants on a structure for

these, with different styling, and those styles had hacks that made dark

mode support difficult to properly add.
In an effort to make this more maintainable, and to add dark mode, we

now have a new CSS component called rb-c-review-comment-thumbnail,

which is responsible for styling the thumbnail area for a comment. This

includes the header (usually a filename and revision) and any thumbnail

contents (an image, part of a document, or a diff).
There are still hacks involved. In particular, we still treat diffs

specially. Instead of providing its own header, this component uses the

diff's filename header. It also works with the expand-controls-on-hover

logic, when opted into. This all involves some special structure and

rules specific to diffs. However, we still benefit from having all of

this nicely grouped together with the base header and content styles,

and all in one component instead of scattered throughout other CSS

files.
Styling has also been fixed up for some of the types of content we

render. Diffed text files, for instance, no longer attempt to override

borders for everything. This will benefit the review UI as well.

Tested in light and dark mode for all comment types, both draft (in the

Review Dialog) and published (in reviews).
Tested diff expansion in light and dark modes in both the Review Dialog

and in published reviews.
Python and JavaScript unit tests pass.