PDFBox Checboxes

I receive a PDF from 3. Party and open it inside my Web service via PDFBox. I then go ahead to manipulate this Checkbox: I wanted to change the Tick mark Symbol to a Caro and the color of that Tick mark Symbol to be light blue.

I succeeded and in general the checkbox now shows a blue Caro if checked, but there is one flaw.

Whenever I click on the Checkbox (during the event 'onClick'), it still shows a black Cross instead of a blue Caro. What did I do wrong? Is there a flag in the Dictionary that determines styles when somebody clicks on the Checkbox?

Is there a way to check whether the Checkbox itself is flawed? I do not rule out, that the 3. Party created the Checkboxes wrongly. Unfortunatly I can not excess the 3. Party right now, so I have to fix the Checkbox inside my Webservice.