Sticky Navigation to the Stella Template

Site URL: https://scarlet-parrotfish-ntsp.squarespace.com/

My client wants a sticky navigation bar at the top of their site-using the Stella template. I don't have experience with code but I found this code on another blog: 

.Header, .Mobile {
  width: 100%;     /* for IE and Edge */
  position: fixed; /* for IE and Edge */
  position: -webkit-sticky; /* For Safari 6.1+ */
  position: sticky; /* Latest browsers */
  top: 0;
  z-index: 4;
}

When I tried that my drop down menus for each of tabs were no longer available.  Like I could see them but then I couldn't get the mouse to click on them because the navigation bar was "stuck" and would be in the way I guess??

Not sure if anyone has had this issue or not?

Hi. What is password?