There is one to make the popup.html border-radius that to add another Div container to popup HTML, set the body background to none and give the background color to the div container. After that, you can give the radius to the container.
e.g
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-css -->
body {
background: none
}
.container {
background: green;
border-radius: 10px;
width: 50%;
height: 100px;
text-align: center;
}
<!-- language: lang-html -->
The content is here....
<!-- end snippet -->