Reduce Padding Space on Mobile Above Header

Looks like you added this custom CSS to your site:

header#masthead {
  padding-top: 75px;
  position: relative;
}

You can wrap that in a media query so it only applies to the minimum screen size of your choosing, for example:

@media screen and (min-width: 800px) {

  header#masthead {
    padding-top: 75px;
    position: relative;
  }

}

You can adjust the min-width as you like, to restrict the change to screens meeting that minimum width.