You don't need var rths = app.runtimeHighight you just need app.runtimeHighlight = false; It should work. Does it work for you? Perhaps the client is viewing the form in a PDF viewer that does not support the script (web browser, etc.). Keep in mind that this script changes the application-wide setting for the user, not just the form. The user might wonder what happened to the highlighting if they prefer that setting.
This is Acrobat's or any other PDF viewer's method of helping the viewer to distinguish between form fields and a general white space.
You can try using scripts or removing the background from the properties, but when the filler gets into Fill & Sign mode, the form fields would be highlighted again to help the viewer.
If this is not what you were looking for, please feel free to share your feedback here: https://adobe.ly/41f0XtO to ensure it reaches the development team for review and future changes.
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