Use same pre background color, highlighted or not

Without this change I found that code blocks without syntax highlighting had the Solarized light background color (`base03` I believe), but code blocks with syntax highlighting had a light silver background. In particular, the code blocks that received the Solarized color were created with the plain Markdown style of indenting the block rather than via backtick fences or the `{% codeblock %}` syntax.

I prefer the silver background and, based on your example page, I believe that's the intent. Of course, please feel free to ignore me if it's not :)

I don't really know SCSS, so I'm not confident whether this was the correct way/place to make the change but it seems to work.