Thanks a lot.
I finally got it !
On the one hand I didn’t use the color section element as you said, on the other hand i used av-submenu-container instead of the onepage-submenu !
That CSS did the trick :
#top .av-submenu-container .active-menu-item,
#top .av-submenu-container .active-menu-item a {
background: #000 !important;
color: #FFF !important;
}

#top .av-submenu-container .av-subnav-menu > li > a {
border-left-style: none !important;
border-left-width: 0px !important;
}

Best regards
Simon