Sticky navigation covering page content

Site URL: https://jewishforgood.squarespace.com/

PW: JewishForGood

I put this code into my CSS to make my navigation sticky and with a shadow effect. It works well, but it reveals the footer background color (?). I can adjust the last part (.Content-outer...) so that the background of the footer doesn't show, but then it covers up some of my page content. 

Any ideas? 

Thank you!

//Fixed Header
.Header--top {
    width: 100%;
    position: sticky;
    position: -webkit-sticky;
    display: block;
    top: 0px;
    left: 0;
    z-index: 9999;
  -webkit-box-shadow: 0px 3px 15px rgba(100, 100, 100, 0.49);
-moz-box-shadow: 0px 2px 12px rgba(100, 100, 100, 0.49);
box-shadow: 0px 2px 12px rgba(100, 100, 100, 0.49);
}
.Content-outer {
margin-top: 70px;
}