Add display custom CSS to thumbnails

This only has an effect if the `display` variable exists in template context where the thumbnail is placed.

OpenShow/slides/templates/slides/iframe-slide-thumbnail.html

@@ -4,6 +4,7 @@
     <link rel="stylesheet" href="{% static 'slides/slide.css' %}">
     <style>
         {{ slide.get_theme.css }}
+        {{ display.custom_css }}
     </style>
     <title>Slide {{ slide.pk }}</title>
 </head>