Gallery short code custom style

Job: change the default style for [gallery] short code
Problem: cannot do this via function in function.php file

function change_gallery_css()
{
	return "<style type='text/css'>
		.gallery img {border: 10px solid #cfcfcf;}
		</style>";
}
add_filter('post_gallery', 'change_gallery_css');

I tested the 'gallery_style' tag instead of 'post_gallery' with no luck.

Please point my mistake.
Thanks in advance for any help.