This theme implements a built-in Jekyll feature, the use of Rouge, for syntax highlighting.
It supports more than 100 languages.
This example is in C++.
All you have to do is wrap your code in a liquid tag:

{% highlight c++ linenos %}
code code code
{% endhighlight %}

The keyword linenos triggers display of line numbers.
Produces something like this:

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int main(int argc, char const \*argv[])
{
    string myString;

    cout << "input a string: ";
    getline(cin, myString);
    int length = myString.length();

    char charArray = new char * [length];

    charArray = myString;
    for(int i = 0; i < length; ++i){
        cout << charArray[i] << " ";
    }

    return 0;
}