For those of us who use this in a real Magic Mirror ;) there’s no need to have any margins (top, left, right, bottom) and it is great to use all pixels of the display. If you’re with me… use the following in ~/MagicMirror/css/custom.css:

body {
    margin: 0px;
    position: absolute;
    height: 100%;
    width: 100%;
}