html, body {
	padding: 0; margin: 0;
    height:100%;
}

body {
	background: #FFF;
}
body[dark] {
	background: #000;
}
iframe {
	width: 50vmin;
	height: 50vmin;
	position: fixed; /* or absolute */
	top: 50%;
	left: 50%;
	transform: translate(-50%, -50%);

	
	-webkit-user-select: none; /* Safari */
	-ms-user-select: none; /* IE 10 and IE 11 */
	user-select: none; /* Standard syntax */
}