The Diff Revision, Commits, and Files section headers in the diff viewer

page had a dark grey background, which conflicted with the background

and didn't match the styling of the rest of the page.
We unfortunately don't have any standard classes for these that we can

depend on. This should be addressed, but not for this release, as it'd

introduce too many structural changes that aren't worth fixing up right

now.
In the meantime, we're now referencing the field label styling via CSS

variables. We're also using the proper font size for section labels,

which is smaller than we had before, but more consistent with the rest

of the page.

Tested the review request and diff viewer pages in light and dark mode,

and compared the styling to the other section headers.