Hi,
For current in rox_menu.css we had applied like this if you want to do it by less then your is the way to do.Make sure with the main li class.
Css code
.gf-menu.l1 li:nth-child(1){background:#e99a1e;}
.gf-menu.l1 li:nth-child(2){background:#b01a40;}
.gf-menu.l1 li:nth-child(3){background:#46aa1f;}
.gf-menu.l1 li:nth-child(4){background:#408afa;}
.gf-menu.l1 li:nth-child(5){background:#d24726;}
.gf-menu.l1 li:nth-child(6){background:#e99a1e;}
.gf-menu.l1 li:nth-child(7){background:#46aa1f;}
.gf-menu.l1 li:nth-child(8){background:#b01a40;}
.gf-menu.l1 li:nth-child(9){background:#408afa;}
.gf-menu.l1 li:nth-child(10){background:#d24726;}
.gf-menu .dropdown ul li{background:none !important}
.gf-menu.l1 li:nth-child(1) div.dropdown{background:#e99a1e;}
.gf-menu.l1 li:nth-child(2) div.dropdown{background:#b01a40;}
.gf-menu.l1 li:nth-child(3) div.dropdown{background:#46aa1f;}
.gf-menu.l1 li:nth-child(4) div.dropdown{background:#408afa;}
.gf-menu.l1 li:nth-child(5) div.dropdown{background:#d24726;}
.gf-menu.l1 li:nth-child(6) div.dropdown{background:#e99a1e;}
.gf-menu.l1 li:nth-child(7) div.dropdown{background:#46aa1f;}
.gf-menu.l1 li:nth-child(8) div.dropdown{background:#b01a40;}
.gf-menu.l1 li:nth-child(9) div.dropdown{background:#408afa;}
.gf-menu.l1 li:nth-child(10) div.dropdown{background:#d24726;}
Thanks