add a link from callout docs to theming reference

Author: Kevin Zuniga Cuellar

docs/tutorialkit.dev/src/content/docs/guides/creating-content.mdx

@@ -165,6 +165,8 @@ Remember to save your work frequently to avoid losing any changes.
 Remember to save your work frequently to avoid losing any changes.
 :::
 
+To customize the styles of a callout, check out the [theming reference](/reference/theming/#callouts).
+
 
 ### Code blocks