Curved background

<div class="container">
...
</div>
.container {
/* Background color */
background-color: rgba(0, 0, 0, 0.3);
/* You can use gradient background color such as
background: linear-gradient(rgba(0, 0, 0, 0.2) 0%, rgba(0, 0, 0, 0.1) 100%); */
/* Curved corners */
border-bottom-left-radius: 50% 40%;
border-bottom-right-radius: 50% 40%;
}

See also

Diagonal section