The unique shape at the top of the page is achieved by using a purple overlay on the header image, creating a curved separation effect. You can view the actual file here.
This technique is commonly used to have a background that covers the entire page, with a separate background specifically for the header and/or footer. Depending on the website's style, this may involve incorporating interesting design separations, such as utilizing .png files to create a faded transition between the header/footer and main background.
There are various methods to accomplish this effect, and different approaches may be preferred by others. In the example site provided, they utilized the background
property in their CSS for both the body and header elements to apply the images. An excerpt from their CSS file shows:
body {
padding:0;
margin:0;
background:#4b1259 url(images/tile-bg.gif) repeat-x;
width:100%;
display:table;
font:16px/22px Georgia;
color:#ffffff;
text-align:left;
}
.header-bg { //this is where i found them adding the header image.
margin-bottom:20px;
background:url(images/header-bg.gif) no-repeat center top;
}