Edit fill and stroke in Form fields
I can't find a way to make a permanent change to appearance of form fields. It will keep having the blue color on top at the end user. In the properties panel it is shown as no fill and stroke. I can see a lot of people asking for this feature - but no solution yet. See https://forums.adobe.com/thread/2467569
Seems like the text field has “Allow Rich Text Formatting” enabled. You can try Cmd + E on MAC and Ctrl + E On Windows to pull the toolbar, and save.
I hope this helps