docs: improve readability for people with poorer eyesight

My eyesight is not in good shape, which means that I have difficulty
reading the online Linux documentation.  Specifically, body text is
oddly small compared to list items and the contrast of various text
elements is too low for me to be able to see easily.

Therefore, alter the HTML theme overrides to make the text larger and
increase the contrast for better visibility, and trust the typeface
choices of the reader's browser.

For the PDF output, increase the text size, use a sans-serif typeface
for sans-serif text, and use a serif typeface for "roman" serif text.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>