bug: theme support in iframe is reverted after agent moves markdown cells

It looks like the theme support in the Markdown iframe is reverted when the agent moves the cell around in the document. The end state I found myself in is below. Reloading the app resolves the issue.

<img width="766" height="944" alt="Image" src="https://github.com/user-attachments/assets/289b247a-9cb8-47dc-a8c2-c46bd8f349b7" />