Zerif lite Menu hidden

Hi,

I am using Zerif-lite theme and its really great. Right now my menu is hidden when viewing on the mobile view. When I click on the menu toggle button, I can see small portion of menu behind the master slider slideshow. I know it has something to do with z-index or something. I have tried several things. Could anyone help me on this ? I also want the menu toggle button to appear when the user scrolls down. The website I am working on is http://www.bg-hairsalon.com