Improve CSS and load fonts from https

Some rules [borrowed by Bedrock](http://mozweb.readthedocs.org/en/latest/css-style.html):
* One rule per line.
* Lower case for hex colors, short form if possible, don’t use web color names (some of them are also not included in CSS 2).
* No quotes in url(), only single quotes where necessary.
* Add Arial as fallback before the default sans-serif font
* Load Open Sans * from https (fix issue #321)