I do not understand the exact problem.
If you don`t want the whole width of the page to be white just give the fieldset a width and so the background color of the page will remain green.
#fieldset {
background-color: #FFFFFF;
width: 100px;
height: 150px;
}
i made an example: http://jsfiddle.net/aKGmc/2/ if this does not help you please upload a jsfiddle with it so i can take a look at the problem