Update CHANGELOG.md

Author: keveleigh

org.mixedrealitytoolkit.theming/CHANGELOG.md

@@ -8,6 +8,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 * Added serializable theme item data types for booleans. [PR #1130](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1130)
 * Added built-in binders for skybox materials and behaviour enabled states. [PR #1130](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1130)
+* Added edit mode tests for validating custom theming types and their dependencies. [PR #1133](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1133)
 
 ## [1.0.0-pre.1] - 2026-05-20