Make code blocks easier to read - gray background

[ozmium]

Change the CSS style for "pre" code blocks. Use a gray background with dark-gray border:

Old:

pre {
 background-color:#ffffff;
 border:1px solid #e5e5e5;
}

New:

pre {
  background-color:#eaeaea;
  border:1px solid #b7b7b7;
}

This is closer to the standard GitHub/Bitbucket style, and increases readability, and makes the rendered document look better.

Preview of Rendered Markdown