Update theme for inline code and code blocks

[WillB97]

The current theme makes the inline code blobs looks very similar to regular text and certain highlighting has poor contrast.

[PeterJCLaw]

From a quick look it seems this is due to the background is provided by the syntax highlighter's CSS, and then only for code blocks (rather than inline). We could probably change this, however doing so in a way which doesn't interfere with the highlighter could get interesting.