Hi bimba1984,

Add this CSS code in Dashboard > Forums > Settings > Styles > "Custom CSS Code" textarea. 

@media screen and (max-width: 620px){
    #wpforo #wpforo-wrap #wpforo-menu .wpf-menu li {
       background: #FFB472;
    }
}
@media screen and (max-width: 620px){
   #wpforo #wpforo-wrap #wpforo-menu li {
       border-right: #FFB472 1px solid;
   }
}

Thank you so much @Sofy, it works fine!