All variables are used in the default template.
The code in the default theme having to do with custom fields is this portions:
{% for field in custom_fields %}
<tr>
<td colspan="99">
<div style="font-weight: bold; padding-top: 20px">{{ field.label }}</div>
<div>{{ field.text | newline_to_br }}</div>
</td>
</tr>
{% endfor %}
The contents of a custom field are in field.text.
Themes are created using Liquid.