Added missing default prettyprint color

If the javascript pretty-printing does not run, the code samples were left in an unreadable color scheme: dark grey on black. 

They will now be white on black.

This is in response to a google group thread: https://groups.google.com/d/msg/vertx/CuskZzuKzwc/EzWzTh0CdF8J

This has been copied over from the old site repo.

Eclipse CLA signed by `danny@kichmeier.us`