Close Captioning Tab “box”

Hi everyone, we have a few keyboard users that would like to be able to more easily see the element that they have tabbed to. They are not visually impaired; this is just a learner preference. I’ve tried a few things but I cannot seem to change the thickness or color of the highlight box. I am using Captivate 2017. Please advise if anyone has any suggestions. Thank you, Sherry