Tweak a bit the default stylesheet:

• Remove background color from <screen> elements. It makes them stand out too much. Some pages have many <screen> elements, listing shell commands and their output. Having a dark background seems a bit too distracting.

• Keep the background color for <programlisting> elements, but make it a bit lighter and add a thin border that is slightly darker than the highlighting background.

• Change the width of <example>, <note>, <warning> and other block elements to use a percentage of the page width instead of a hardcoded pixel-based length.

• Add Wikipedia-inspired coloured backgrounds and thin borders around <example>, <note>, <warning> and tip elements.

Reviewed by: blackend, gavin, rene
No objections: freebsd-doc