docbook: change css style

A handful of random personal preference:

- Force sans-serif for the text.
- Quote code sample literal inside a single-quote pair.
- Show emphasis in blue-green italics.
- Do not use itarlics for term definition, but show them in navy.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>