To accomplish this look for your main navigation menu in the Storefront theme, you can go to Appearance | Customize | Additional CSS and add this code:

.menu-item a{
	padding-top:0px !important;
	padding-bottom:8px !important;
	color:white !important;
	font-size:20px;
	background-color:navy;
	border-radius:10px;
	padding-bottom:2px !important;
	border:2px solid black;
	margin-left:4px;
		margin-right:4px;
	box-shadow:4px 4px 4px silver;
			margin-bottom:8px;
}
#menu-main-menu{
	text-align: center;
	margin-bottom:15px;
	background
}
.menu-item a:hover{
	background-color:blue !important;
}
.sub-menu{
	background-color:transparent !important;
}
.menu-toggle{
	background-color:navy;
	text-align: !important;
	font-size:20px;
	background-color:navy;
	border-radius:10px !important;
	border:2px solid black !important;
	margin-left:4px;
	margin-right:30px;
	box-shadow:4px 4px 4px silver !important;
			margin-bottom:8px;  	
}
.menu-toggle span{
	padding-right:18px !important;
}