Allow variables for <accent> for Overlay Alert effect

Initial thought: `$chatUserColor`
Additional: custom variables , so highlight color can be changed based on thresholds (raidcount, tip amount, etc)

<img width="595" height="1281" alt="Image" src="https://github.com/user-attachments/assets/612c4e0a-50e9-4538-8781-87163a8b3aeb" />