Use callout without custom Jekyll plugin.

docs/4.0/getting-started/theming.md

@@ -272,9 +272,10 @@ Here's how you can use these in your Sass:
 
 [Color utility classes]({{ site.baseurl }}/docs/{{ site.docs_version }}/utilities/colors/) are also available for setting `color` and `background-color`.
 
-{% callout info %}
+{% capture callout %}
 In the future, we'll aim to provide Sass maps and variables for shades of each color as we've done with the grayscale colors below.
-{% endcallout %}
+{% endcapture %}
+{% include callout.html content=callout type="info" %}
 
 ### Theme colors