To achieve this particular design, I would utilize two distinct elements.
The first element would be a black background that is fixed and stretches vertically and horizontally.
On top of that, I would include my "classic" window with rounded corners.
<body>
<div></div>
<div>
<header></header>
</div>
</body>
... and here is the CSS styling :
body > div:first-of-type {
position : fixed;
top : 0;
left : 0;
bottom : 0;
right : 0;
z-index : -1;
background-color : black;
}
body > div:nth-of-type(2) {
background-color : white;
position : fixed;
top : 0;
left : 0;
bottom : 0;
right : 0;
z-index : 1;
border-radius : 25px;
}
body > div:nth-of-type(2) > header {
height : 25vh;
background : linear-gradient(to bottom, #54c3ff, #ddf7ff);
border-radius : 25px 25px 0 0;
}