Can't add padding to image in feature section box

I have three feature boxes, each with an image. I have put a bottom border on each image to separate it from the heading below. However, the padding sits right against the image so I want to add 10px of padding to the bottom of the images. There’s no option for it in cornerstone. I’ve tried adding custom css as well with no effect.

.x-feature-box-graphic {
padding-bottom: 10px;
}

also tried adding it to the class on the img itself

img.square {
padding-bottom: 10px;
}

Any ideas what’s going on?

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