Add `theme` parameter to broadcast message API

What does this MR do and why?

Adds the theme property to the broadcast messages API. This field was introduced several milestones ago, yet was not added to the API at that time.

Closes #498900

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.