Sticky Header is Transparent But Can't Change Color or Opacity

Hi, I'm using GP Premium and have been able to edit most of the Sticky Header but I cannot locate where to make changes to the transparency color as well as its opacity. I'm able to make changes to most of the header but when a reader scrolls down the page and the sticky header becomes transparent it turns an opaque purple and we cannot change this color nor the opacity. How do we make these changes? Thanks in advance.