Removing border below header / menu

There is some shadow below the header/menu of my site. I would like to remove it, but it won’t go.
I tried various things in the CSS (found them also here on the forum), but nothing works.
The problem is that when I insert a avia slider, it will be placed on top of this line. On wider screens, left and right of the 1500 pixels wide image, the line appears. That looks stupid.

Please tell me how to remove this line / shadow.

What I tried (and none of them worked):

#header_main {
border-bottom: none !important;
}

.avia-layerslider.avia-shadow {
box-shadow: none !important;
}

#header {
border-bottom: none !important;
}

Kind regards!