how to remove shadow under top menu

hello,

how can i remove the shadow under top menu and rounded bottom ?

i want my top menu to be as the top menu on your demo site.

i’ve seen in faq to add a custom css

#custom h2.header-title:before,#custom h2.header-title:after { display:none; }

for do it on archives titles and it works perfect but not for top menu.

Thanks in advance for your help