No border on aligned images/media?

When I insert an image and use “aligncenter” in the code (or alignleft or alignright), a grey border is put around the image. I went and found where these are defined in the stylesheet:

img.alignleft,
.alignleft img,
img.aligncenter,
.aligncenter img,
img.alignright,
.alignright img {
	box-sizing: border-box;
	padding: 5px;
	border: 1px solid #EEE;
}

However, removing any or all of the three lines within the brackets (in my child theme) does not get ride of the border. Any ideas why? The only way I’ve been able to hide the border has been to change the border to white, but I’d rather know how to just get rid of it all together.

Any thoughts? Thanks so much.