Other reviewers have already covered the CSS well.
As for this HTML:
<h3>Color Reference</h3> <code class="pln ">Plain text</code><br /> <code class="str">"String content"</code><br /> <code class="kwd">Keyword</code><br />
I think <br/> is not a good practice in general.
I recommend to get rid of those,
and do something like this instead:
<h3>Color Reference</h3>
<div class="examples">
<p><code class="pln ">Plain text</code></p>
<p><code class="str">"String content"</code></p>
<p><code class="kwd">Keyword</code></p>
</div>
In the <head> of your tester page (decidedly not in the prettify CSS file),
add custom <style> to format examples.p appropriately.