For syntax highlighting, use fenced code blocks optionally followed by the language syntax you want:

    def foo
      puts 'foo'
    end

Fenced code blocks require a blank line before and after.

If you're using an HTML file, you can also use the highlight command with Liquid markup:

{% highlight ruby %}
    def foo
      puts 'foo'
    end
{% endhighlight %}

The theme has syntax highlighting configured in the configuration file as follows:

highlighter: rouge