Сделать Pop up прозрачный по середине

body{
background: linear-gradient(135deg, darkblue, darkred);
}
#x{
display: block grid;
grid-template-columns: repeat(5, 1fr);
position: fixed;
height: 100px;
bottom: 0;
border-radius: 25px;
background: radial-gradient(circle at 50% 0px, transparent 60px, transparent 60px, lightgreen 61px, lightgreen, green);
}
#x > *{
text-align: center;
}
#x > *:nth-child(3){
display: inline-block;
position: relative;
height: 100px;
width: 100px;
border-radius: 50%;
top: -50%;
background: linear-gradient(75deg, lightgreen, green);
}

<div id='x'>
<h1>+</h1>
<h1>+</h1>
<h1>+</h1>
<h1>+</h1>
<h1>+</h1>
</div>