Screenshot helped.
Indeed for last menu items you used extra CSS class name, and for others not. So in general, it was your small mistake.
Add that Custom CSS
.offcanvas-menu .offcanvas-inner ul.menu ul {line-height: 3;}
.sp-megamenu-parent .sp-dropdown li.sp-menu-item > a,
.offcanvas-menu .offcanvas-inner ul.menu > li > ul li a { color: white !important;}
And remove that Custom CSS that was added by you. It will be not needed anymore.
.menu_principale {
color: #ffffff !important;
line-height: 30px !important;
}
And you own me a Pizza ;)