More color contrast non-compliance

[nigelmegitt]

See #391 for additional links.

The color of code blocks is #C83500 which on the background color of permitted-deprecated blocks (class="deprecation") of #E6E6FA is not WCAG compliant according to snook.ca.

In fact the color of those code blocks is never WCAG 2 AAA compliant even on a white background, but we should at least target AA compliance, which can be achieved by setting the background color to #EBEBFE, which I propose we adopt.

In example fragments, the color of comments in the class hljs-comment is #999988 which on an example background color of #FFFFC8 is non-compliant. There doesn't seem to be any good reason to use this comment class so I propose fixing by just not using it. Then the color of #333333 on that background is compliant.

I've not done an exhaustive check (I suspect there's a tool for that...) but those two caught my eye.

[nigelmegitt]

To clarify, the first proposal is to set the background-color of permitted-deprecated blocks to #EBEBFE.