Search result highlighting is broken in docs

[ashb]

If you search for a string in the docs then the occurrences of the search string on the page will be wrapped in <span class="highlight"> -- currently we don't style this.

The "default" style for the previous theme was to set the background colour for this tag -- we should just pick a "nice" colour.

[ryw]

@ryanahamilton maybe you could fix this