Don Gilbert

@dadarya0 The customHtml not populating after switching the template _without_ opening the builder is due to an issue with the GrapesJS plugin; the plugin overwrites Mautic's core `setThemeHtml` method, and...

@dadarya0 I discovered in the GrapesJS builder service commented out code which handled setting the customHtml from the provided mjml when changing themes. I uncommented and modified it, as well...

@escopecz @kuzmany Please review the content changes I made in the PR body as well as the linked documentation.

The last two commits [a52793e](https://github.com/mautic/mautic/pull/14812/commits/a52793eb14c2099fb9c2e2e1c390325c7d345ef2) and [05093ef](https://github.com/mautic/mautic/pull/14812/commits/05093efb2a8fc4e9adb9e3dec74dd143d7a55e79) were pushed to address the error below which came up while running test cases. ``` [2025-07-30T18:31:35.045343+00:00] mautic.CRITICAL: Uncaught PHP Exception TypeError: "Cannot assign...

Thanks for the thorough review @escopecz! I'll push an update to address the issues.