Gap between fieldset and div element?

[KennyFan382]

On my webpage I have got an annoying gap that seperates my background color from the edge of my fieldset. I am trying to find a way to fill it in using css, it appears to not be working, Here is an image with the problem and the css for the fieldsets and the div element. (#left and #right are the fieldset elements, they both needed different margins to be placed on each different side to the page).

div {
    background-color: black;
    border: none;
    margin: auto;
    float: top;
    position: relative;
    bottom: 28px;
    right: 13px;
}

#left {
 margin-right: 400px;
 border-color: black;
}

#right {
    margin-left: 400px;
    border-color: black;
}