Siteleaf uses the popular Liquid syntax for themes. If you can write HTML, you’ll have no problem using Liquid.
Here’s how a simple template might look:
<html>
<head>
<title>{{site.title}} | {{title}}</title>
</head>
<body>
<h1>{{title}}</h1>
<article>{{body}}</article>
{% if parent %}
<p><a href="{{parent.url}}">← Go back</a></p>
{% endif %}
</body>
</html>
As you can see, there are two types of markup in Liquid:
1) Output markup (which may resolve to text) is surrounded by
{{ matched pairs of curly brackets (ie, braces) }}
2) Tag markup (which cannot resolve to text) is surrounded by
{% matched pairs of curly brackets and percent signs %}
If you are new to the Liquid syntax, a good place to start is Liquid for Designers.