The text review UI has been updated to support dark mode styling. The

text content already supported dark mode, but the custom tabs and the

overridden styles for the filename header did not.
We now use rb-c-tabs for the tabs, helping that fit in with much of

the UI. We also use the default diff viewer header styling that we

inherit for the filename information.
Tabs had to be updated to ensure we correctly override the color and

text-decoration for the label, in case <a> is used (which is the

case here).
With this, we now have full dark mode support for text and Markdown

review.

Tested in light and dark modes with single revisions, diffs, plain

text, and Markdown files.