The application doesn't allow you to create rounded corners, but after the page has been compiled and saved, you can edit the files with a HTML or a text editor and then add the following code for the element you want:
border-radius: 5px;
-moz-border-radius: 5px;
-webkit-border-radius: 5px;
border: 2px solid #800000;
Adjust the color, radius and the line weight in px.