Sidebar navigation shows up right in stead of left.

change this 

.header--menu-open .header-menu {
    opacity: 1;
    visibility: visible;
    width: 50%;
    margin-left: 50%;
    box-shadow: 5px 5px 15px #000;
}

to this 

.header--menu-open .header-menu {
    opacity: 1;
    visibility: visible;
    width: 50%;
    margin-right: 50%; //this needs to be right for it to sit on the right side.
    box-shadow: 5px 5px 15px #000;
}