Hello,
Try to add the next code in Theme Options > Theme Custom CSS > Global custom CSS:
.item-design-mega-menu {
position: static;
}
.header-top, .header-main, .header-bottom {
position: static;
}
.item-design-mega-menu .nav-sublist-dropdown {
left: 0 !important;
margin: 0 !important;
width: 100% !important;
}
.item-design-mega-menu .nav-sublist-dropdown:before {
content: '';
display: block;
position: absolute;
left: 0;
right: 0;
height: 40px;
bottom: 100%;
}
Regards