There are several ways to achieve this:
Use Jekyll _includes
along with parameters. Simple example would be to style an alert block.
<!-- /_includes/alert.html -->
<div class="alert alert-info" role="alert">
{{ include.content }}
</div>
And you’d use it in your content like so:
Some content here.
{% include alert.html content="Your alert text" %}
Some more content.
Write a custom Jekyll tag (aka a plugin). This would be similar to the include method above but you you’d have something like:
{% alert %}
Your alert content here.
{% endalert %}
Or you can Augment Kramdown written Markdown with selectors to apply CSS classes to a block of text styling it however you need.
Your alert text
{: .alert .alert-info}