Add info about new theme colours to changelog

Author: michalkleiner

CHANGELOG.md

@@ -4,6 +4,13 @@ This is the Developer Changelog for Matomo platform developers. All changes in o
 
 The Product Changelog at **[matomo.org/changelog](https://matomo.org/changelog)** lets you see more details about any Matomo release, such as the list of new guides and FAQs, security fixes, and links to all closed issues. 
 
+## Matomo 5.6.0
+
+### New Features
+
+* Themes can now customize the focus ring colors using `@theme-color-focus-ring` (used globally) and `@theme-color-focus-ring-alternative` (used in header navigation on solid background).
+
+
 ## Matomo 5.4.0
 
 ### Authentication changes