there are stylesheet links in facebook’s instant article example articles:
<head>
<meta charset="utf-8">
<link rel="canonical" href="http://instantarticles.fb.com/example3">
<link rel="stylesheet" title="default" href="#">
<title>Using Captions in Instant Articles</title>
<meta property="fb:article_style" content="Test Article Style">
</head>
the publishers above probably use stylesheets of their own. i tried putting some font definitions in a stylesheet and added it to my instant pages RSS template, but it didn’t work. without proper documentation there’s probably no other way than trial and error.