Format WP Gallery Image Border

I’m trying to format WP gallery image borders in TwentyTwelve via CSS exactly the same as they appear if posted to a page without the gallery, i.e no border and a 2 or 3px box shadow and radius. Setting border to none doesn’t seem to remove it – it seems to just make it transparent. The default box shadow and image caption remain in their original position. See e.g. the images in Marcy Diaz’s article on removing gallery image borders in TwentyTwelve:

https://amethystwebsitedesign.com/remove-border-on-wordpress-gallery-images/

I can remove the default box shadow following mheltone’s posts:

https://ww.wp.xz.cn/support/topic/is-there-a-way-i-can-remove-a-shadow-border-around-my-images

But then I don’t see how to put a box shadow at the edge of the image itself. I’ve inspected the code in Firefox and searched this forum but I’m stuck. I’d be grateful for any suggestions. Thanks