Border under main menu problem

Hi,

i am trying to set up the theme and want to remove the following bottom border:

I tried removing it, but so far it didn’t work.
The highlighter under the main menu can stay, but how do i change or remove the other bottom border?

At the moment i got the following css:

#header_main_alternate {
border: none;
}

#main_color container_wrap fullsize {
border: none;
}

#header_main {
border: none;
}