If you inspect carefully the code, you will notice that on the body element the background is:
background: #000000 url("../img/background.png") repeat-x scroll left 30px;
This means that there is solid black color and one image also for background. So the top part of this image is the white line. If you want to remove it this is what you need to do: first find the code in the file 'styles.css' (which you can find here: "~Themes/NeoFashion/Content/css/styles.css") and remove the code that set the image for background, and leave just the solid color. This is how the code should looks like after this change:
body { background: #000000; color: #c0c0c0; font: 15px Sylfaen,sylfaen_ff,serif; }