Unfortunately @juvyjabian, I have not found a way!

I attempted this take:

 <style>
@import url('https://fonts.googleapis.com/css?family=Roboto');

.font {
    font-family: 'Roboto', sans-serif;
}
</style>

<p>Normal Text.</p>
<p class="font">Roboto</p>

..., but it would not render.

Still exploring!
Try this article, though: https://guides.github.com/features/mastering-markdown/