gh-123430: Add dark mode support to pages generated by http.server (#123475)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>