use Palatino as base font; simplify heading style
use Palatino as base font; simplify heading style

See #25 for details. These changes make the stylesheet closer in spirit to raw HTML, by:
- using a single font throughout
- making the contrast reduction subtler
  (and, as a bonus, now using the components of classic
  [web-safe colors](https://en.wikipedia.org/wiki/Web_colors#Web-safe_colors),
  i.e. 0, 3, 6, 9, C, F for each of the RGB)
- simplifying the styles for the headings,
  some of which were needed due to the previous font family
waldyrious/unstyle