Hi,
I think I have the solution of your problem:
Go to line 1033 in the CSS file that you've attached and change to this:
.sticky {
/*
position: fixed;
z-index: 999;
top: 0;
box-shadow: 0 4px 0px rgba(0, 0, 0, 0.1);
*/
}
I hope this is what you're looking for :)
Regards,
Aleksandar