centering the menu across the full container

Hey!

Try adding this code to General Styling > Quick CSS:

#header_main_alternate .container {
    width: 100%;
    max-width: 100%;
}

#header_main_alternate ul {
    display: flex;
    justify-content: space-between 
}

#header_main_alternate ul li{width: 100%;}

.html_header_top #top .av_menu_left .main_menu{
    left: 0;
}

Cheers! 
Josue