The normal view’s iframe has parent elements with background-color that is commented out seemingly. And a parent with background-color: but it’s got 0 width so doesn’t make any difference. Then the body has background-color:#f6f6f6 which is a light grey. This shows through the iframe I think, so the background of the generator looks grey too.
If you go from there and click “edit” it looks different in the preview. And click “fullscreen” and it still looks different in full-window view.
Here: the iframe has background-color:white on it. The parent #output div has the same. The parent of that has the same (though weirdly that element does not fill the size of the window or whatever so it looks like a box to the top-left). And from then on it falls through to the top-level body’s grey again.
Ideally, these should not be different for editor-mode view and non-editor-mode view.