Image selector Twenty Twelve

Hi,

I’m trying to add a margin below the blue button that appears on this page https://www.triresources.com/reference-tools/ but can’t do it.

This is the CSS I’m using.
img.aligncenter.wp-image-1173.size-full {
margin-bottom: 1.6rem;
}

I found the selector using Firefox developer tools but it doesn’t seem to work.

I also added “button” as the Image CSS Class but that didn’t work either.

Any advice on how to add a margin below the button would be appreciated.

Thanks.