Render unchanged diff lines as grey instead of yellow

In Gitiles unified diff formatting (HtmlDiffFormatter), unchanged
context lines (starting with ' ') are wrapped in <span class="Diff-change">.
Previously, dark mode support introduced --diff-change styled as #960
in light mode and #fdd663 in dark mode, causing all unchanged lines
in unified diffs to render as yellowish-brown / yellow instead of neutral
grey.

Update --diff-change to #666 in light mode and #9aa0a6 in dark mode so
that unchanged context lines render in grey matching the secondary text
palette.

Add unit tests to verify that unchanged lines are formatted with
Diff-change and that --diff-change resolves to grey colors instead of
yellow in base.css.

Bug: b/562476907
google-bug-id: 562476907
TAG=agy
CONV=f9f02e8a-5730-4ba4-9c5c-f2415cbb4fd7

Change-Id: Ie3c687845b707161f462b5e5cfe10ce481e9fa2b