Attilio, I ran the same but got different results. This is with v0.12 on a Mac.
I created the highlights file.
$ cat content/post/sample.md
+++
date = "2014-12-02T10:51:45-06:00"
draft = true
title = "sample"
+++
## some demo code
{{% highlight c %}}
int c;
if(c) {
return c;
}
{{% /highlight %}}
$
Ran the server.
$ hugo --buildDrafts server
1 of 1 draft rendered
0 future content
1 pages created
0 tags created
0 categories created
in 105 ms
Serving pages from /Volumes/G-DRIVE Data/Sites/playground/public
Web Server is available at http://localhost:1313
Press ctrl+c to stop
The browser looks right.
some demo code
int c;
if(c) {
return c;
}
Home
The generated HTML file looks right.
$ cat public/post/sample/index.html
<!DOCTYPE html>
<html>
<title>sample</title>
<body>
<h2 id="toc_0">some demo code</h2>
<div class="highlight" style="background: #272822"><pre style="line-height: 125%"><span style="color: #66d9ef">int</span> <span style="color: #f8f8f2">c;</span>
<span style="color: #66d9ef">if</span><span style="color: #f8f8f2">(c)</span> <span style="color: #f8f8f2">{</span>
<span style="color: #66d9ef">return</span> <span style="color: #f8f8f2">c;</span>
<span style="color: #f8f8f2">}</span>
</pre></div>
<p><a href="/">Home</a></p>
</body>
</html>
$
What does your HTML file contain?