doc: update stylesheet to match frontpage

- Changed colors to match frontpage as close as possible.
- Links are slightly more horizontally padded as compared before to
  accomodate for the hover effect.
- Slightly reduced the scroll indication height on the TOC.
- The main content is now offset using margin instead of the previous
  border hack.
- remove empty footer that was rendering a dark bar on the bottom of
  each page without any content.

PR-URL: https://github.com/nodejs/node/pull/4621


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>