CSS: fix highlight style

When language is `c`, HTML is the following:

    <div class="highlight">
      <pre>
        <code class="c language-c" data-lang="c">
          <span class="kt">void</span>...
        </code>
      </pre>
    </div>

To avoid `.highlight .c` matches `div.highlight code.c`, change it
to `.highlight span.c`.
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