[Theme: Zerif lite] Fixed menu for mobile (Responsive)

Hello i fixed navigation menu on my website by adding this code :

`.main-navigation {
clear: both;
display: block;
float: left;
width: 100%;
position: fixed;
}

Its working fine on desktop version :-). But not working on responsive mobile menu. How i fixed mobile menu? i adding “position: fixed;” into the .responsive-nav class and .responsive-logo in css but menu is not fixed in responsive version of my site. It is something way how i fixed responsive menu and other header element? The link of my site is : http://www.prc.sk/.
Link to css: http://prc.sk/wp-content/themes/zerif-pro-production/style.css?ver=v1
Thank you 🙂