It's called CSS.
The plain old HTML look is created by the default CSS settings. If you want to change the look, then you need to change the CSS. Find a website that has a look similar to what you want, and look at the HTML source. You will see a lot of CSS near the begining wrapped by STYLE tags. For instance:
<style type="text/css">
input {
border: none;
background: #FFF;
width: 165px;
}
.rounded {
background: url(rounded.gif) no-repeat left top;
padding: 8px;
width: 180px;
}
</style>
In order to get the actual rounded corners you are going to need some images that can cover the sharp corners. In the example CSS it refers to a single image of a box, but generally you will need four corner images, and four separate line images (top, bottom, left, right).
Check this article about creating forms with rounded elements: http://www.picment.com/articles/css/funwithforms/