Style for printing

Hi,

I’m trying to add a printing style but it doesn’t work. I added the code below to my child theme css:

@media print {
/* Here you can add specific styles for the printing */
	body {
		background: #FFF;
		color: #000;
		margin: 0;
	}
	#header, #comments, #sidebar, #footer {
		display: none;
	}
	#content {
		display: block;
	}
}

When I try to print, everything is displayed in the preview, not just the main content. Did I miss something? Should I add the media print in the header? How?