you may do it with the script below. Suppose the name of floating field is "temp". form1.temp.ui.oneOfChild.border.fill.color.value = "255, 255, 255"; You may change the RGB value as per your requirement.
Please refer to the following tutorials for the same:https://experienceleague.adobe.com/docs/experience-manager-65/forms/getting-started/create-first-interactive-communication/create-your-first-interactive-communication.html?lang=en#step-create-form-data-model https://experienceleague.adobe.com/docs...
I would suggest you to use Adaptive Forms(instead of HTML5 forms) using the same XDP template, , as adaptive forms offers Form data model, using which you can easily achieve this use case.
Sorry Mark, but even Windows 10 is also not supported, as per the link. You need to use 64 bit Windows Servers like the versions 2012 R2 or 2016, as per the link above. Windows 10 or Windows 11 are not Windows Servers.