#block-system-main-menu ul.main-menu>li> .sub-menu::-webkit-scrollbar {
    width: 5px;
}

/* Track */
#block-system-main-menu ul.main-menu>li> .sub-menu::-webkit-scrollbar-track {
    background: none; 
}
 
/* Handle */
#block-system-main-menu ul.main-menu>li> .sub-menu::-webkit-scrollbar-thumb {
    background: #000; 
}

/* Handle on hover */
#block-system-main-menu ul.main-menu>li> .sub-menu::-webkit-scrollbar-thumb:hover {
    background: #000; 
}